`find_in_batches` now returns an `Enumerator` when called without a block, so that it
can be chained with other `Enumerable` methods.
Replace Post with Article in getting started guide [ci skip]

- Used Article model instead of Post as it may confuse with 'post'
  requests
- Tried to wrap the guide to 80 chars lines as much as possible.
- Removed unused image
- Fixes #13764
Add CreateMigration action

This Thor-action isolates the logic whether to (over-)write migration and
what is shown to the user. It's modelled after Thor's CreateFile-action.

This solves the issue that removing a non-existing migration, tried to
remove the template-path (#13588).

Related issues: #12674
Correctly send the string given to lock! and reload(:lock) to the lock scope - fixes #13788

As per the documentation at lock!, if the :lock option is a string it should use the given SQL to generate the lock statement.
Merge pull request #13868 from mauricio/bug-13788

Correctly send the string given to lock! and reload(:lock) to the lock scope - fixes #13788
Allow session serializer key in config.session_store

MessageEncryptor has :serializer option, where any serializer object can
be passed. This commit make it possible to set this serializer from configuration
level.

There are predefined serializers (:marshal_serializer, :json_serialzier)
and custom serializer can be passed as String, Symbol (camelized and
constantized in ActionDispatch::Session namepspace) or serializer object.

Default :json_serializer was also added to generators to provide secure
defalt.
Merge pull request #13692 from lukesarnacki/change-default-session-serializer

Allow session serializer key in config.session_store
Update CHANGELOG properly with GH #13692 [ci-skip]
Fixed a bug in AR::Base#respond_to?

Before:

  >> ActiveRecord::Base.respond_to?(:find_by_something)
  NoMethodError: undefined method `abstract_class?' for Object:Class

After:

  >> ActiveRecord::Base.respond_to?(:find_by_something)
  => false
`scope` now raises on "dangerous" name conflicts

Similar to dangerous attribute methods, a scope name conflict is
dangerous if it conflicts with an existing class method defined within
`ActiveRecord::Base` but not its ancestors.

See also #13389.

*Godfrey Chan*, *Philippe Creux*
`enum` now raises on "dangerous" name conflicts

Dangerous name conflicts includes instance or class method conflicts
with methods defined within `ActiveRecord::Base` but not its ancestors,
as well as conflicts with methods generated by other enums on the same
class.

Fixes #13389.
Merge pull request #13450 from chancancode/ar_raise_on_conflict

Raise an error if `scope` or `enum` is about to add a conflicting method to the class

Fixed #13389
Mention find_each in find_in_batches doc [ci skip]
Merge pull request #13877 from marcandre/find_in_batches

Mention find_each in find_in_batches doc [ci skip]
Merge pull request #13201 from marcandre/find_in_batch_enumerator

`find_in_batches` now returns an `Enumerator`

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/lib/active_record/relation/batches.rb
Enhance errors while retrieving database config

Right now if there is an error retrieving database configuration the intent of the error (what the code was trying to do while you got the error) could be more explicit. 

Instead of this error:

```
Invalid DATABASE_URL: nil
(erb):9:in `rescue in <main>'
(erb):6:in `<main>'
/Users/schneems/.rbenv/versions/2.1.0/lib/ruby/2.1.0/erb.rb:850:in `eval'
/Users/schneems/.rbenv/versions/2.1.0/lib/ruby/2.1.0/erb.rb:850:in `result'
/Users/schneems/Documents/projects/rails/railties/lib/rails/application/configuration.rb:98:in `database_configuration'
/Users/schneems/Documents/projects/rails/activerecord/lib/active_record/railtie.rb:41:in `block in <class:Railtie>'
/Users/schneems/Documents/projects/rails/railties/lib/rails/railtie.rb:237:in `instance_exec'
/Users/schneems/Documents/projects/rails/railties/lib/rails/railtie.rb:237:in `block in run_tasks_blocks'
/Users/schneems/Documents/projects/rails/railties/lib/rails/railtie.rb:237:in `each'
/Users/schneems/Documents/projects/rails/railties/lib/rails/railtie.rb:237:in `run_tasks_blocks'
/Users/schneems/Documents/projects/rails/railties/lib/rails/application.rb:339:in `block in run_tasks_blocks'
/Users/schneems/Documents/projects/rails/railties/lib/rails/engine/railties.rb:13:in `each'
```

I propose we issue this error:

```
Cannot load `Rails.application.database_configuration`:
Invalid DATABASE_URL: nil
(erb):9:in `rescue in <main>'
(erb):6:in `<main>'
/Users/schneems/.rbenv/versions/2.1.0/lib/ruby/2.1.0/erb.rb:850:in `eval'
/Users/schneems/.rbenv/versions/2.1.0/lib/ruby/2.1.0/erb.rb:850:in `result'
/Users/schneems/Documents/projects/rails/railties/lib/rails/application/configuration.rb:98:in `database_configuration'
/Users/schneems/Documents/projects/rails/activerecord/lib/active_record/railtie.rb:41:in `block in <class:Railtie>'
/Users/schneems/Documents/projects/rails/railties/lib/rails/railtie.rb:237:in `instance_exec'
/Users/schneems/Documents/projects/rails/railties/lib/rails/railtie.rb:237:in `block in run_tasks_blocks'
/Users/schneems/Documents/projects/rails/railties/lib/rails/railtie.rb:237:in `each'
/Users/schneems/Documents/projects/rails/railties/lib/rails/railtie.rb:237:in `run_tasks_blocks'
/Users/schneems/Documents/projects/rails/railties/lib/rails/application.rb:339:in `block in run_tasks_blocks'
/Users/schneems/Documents/projects/rails/railties/lib/rails/engine/railties.rb:13:in `each'
```
Added some style changes in asset pipeline documentation
Fix documentation of new controller filters api [ci skip]

The api for filters with classes change and the guides weren't updated.
Now the class must respond for methods with the same name as the filter,
so the `before_action` calls a `before` method, and so on.

The method `#filter` has been deprecated in 4.0.0 and has been removed
in 4.1.0: #7560
Merge pull request #13871 from kassio/fix-filters-with-classes-documentation

Fix documentation of filters api with classes [ci skip]
Fixing issue with activerecord serialization not being able to dump a record after loading it from YAML - fixes #13861