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
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
Fixing issue with activerecord serialization not being able to dump a record after loading it from YAML - fixes #13861
Merge pull request #13876 from sowjumn/master

Added some style changes in asset pipeline documentation
Merge pull request #13867 from mauricio/bug-13861

Fixing issue with activerecord serialization not being able to dump a record after loading it from YAML - fixes #13861
Aesthetic
Merge pull request #13879 from schneems/schneems/more-explicit-database-config-error

Enhance errors while retrieving database config
Implement a simple stub feature to use in the Time travel helpers
Alias the original method first to avoid warnings
Use instance method instead of before hook
Change the class and method visibility
Store the singleton_class in a local variable
Use each_value
Merge pull request #13883 from rafaelfranca/rm-time-travel

Alternative implementation to make time travel not dependent on mocha
find_in_batches should not mutate its argument
Modify the session serializer implementation

Rename allowed options to :marshal and :json, for custom serializers
only allow the use of custom classes.
Merge pull request #13888 from rails/session-serializer

Modify the session serializer implementation
docs, `references` is only used with `includes`. Closes #13727.

There is no gain in `referencing` tables that are not used for preloading.
Furthermore it will break if polymorphic associations are invloved. This
is because `references_eager_loaded_tables?` uses all `reference_values`
to decide wether to `eager_load` or `preload`.