Use symbols instead of strings

ActiveSupport::Concern is used all over Rails
This PR will only create 3 new objects as keys are never recreated and
are not subject to garbage collection.

The strings were being uniquely created and garbage collected. I don't
have any performance numbers but this should be better than all of the
GC.
Improved grammar and replaced 'dbs' slang with 'databases'
Do not invoke callbacks when delete_all is called

Method `delete_all` should not be invoking callbacks and this
feature was deprecated in Rails 4.0. This is being removed.
`delete_all` will continue to honor the `:dependent` option. However
if `:dependent` value is `:destroy` then the default deletion
strategy for that collection will be applied.

User can also force a deletion strategy by passing parameter to
`delete_all`. For example you can do `@post.comments.delete_all(:nullify)`
s/plugin_new/plugin

There are historical reasons that the `plugin` command was `plugin_new`, now those are no longer applicable, we should remove the naming edge case from the project. This PR is based off of comments from #11176

ATP Railties
Remove redundant escapes from xml serialization test
Merge pull request #10099 from wangjohn/railties_each_with_config_2nd_attempt

Created way of cloning a Rails::Application instance
Further clean-up of ActiveSupport::Callbacks

In #11195 I noticed a trailing comma in the docs, but I decided to
further clean it up.

What I have done:

* Clean up the trailing comma in the docs and some extra whitespace
  lines.
* Used `Array#extract` options to factor the repetitive pattern of
  `args.last.is_a(Hash) ? ...`
* Renamed the local var `config` to `options` in `define_callbacks`, as
  `options` seems to be the de facto name for the options objects.
* Renamed the local var `l` to `line` in `define_callback` (maybe it
  meant `lambda` in the context) as single `l` may look like `1` in some
  fonts.
Merge pull request #11211 from gsamokovarov/trailing-comma

Further clean-up of ActiveSupport::Callbacks
Merge pull request #11207 from vipulnsward/remove_escapes

Remove redundant escapes from xml serialization test
Using ActiveSupport::Concern instead of plain regular Ruby hooks in railties/lib/rails/generators/migration.rb file
Removed deprecated method default_scopes?
Removed deprecated method scoped
Merge pull request #11203 from schneems/schneems/plugin_new-plugin

s/plugin_new/plugin
Merge pull request #11212 from neerajdotname/default_scopes_q_deprecated

Removed deprecated default_scopes?
Merge pull request #11213 from neerajdotname/scoped-deprecated

Removed deprecated scoped method

Conflicts:
	activerecord/CHANGELOG.md
Removed deprecated methods partial_updates and family

Removed deprecated methods `partial_updates`, `partial_updates?` and
`partial_updates=`
Merge pull request #11221 from neerajdotname/partial_updates_family

Removed deprecated methods partial_updates and family
Merge pull request #11220 from aditya-kapoor/add-concern

Change Rails Migration generator module to use AS::Concern
remove deprecated `assert_present` and `assert_blank`.
Remove deprecated `:distinct` option from `Relation#count`.