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.
Making the rake file for tests easier to read.

I'm defining a new class which modularizes how the `rake test` tasks are
defined and invoked.
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There are historical reasons that the `plugin` command was `plugin_new`, now those are no longer applicable, we should remove the naming edge case from the project.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
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`.
Merge pull request #11226 from senny/remove_deprecated_distinct_option

Remove deprecated `:distinct` option from `Relation#count`.
Merge pull request #11224 from senny/remove_assert_blank_and_assert_present

remove deprecated `assert_present` and `assert_blank`.
remove deprecated `BufferedLogger`.
remove auto-explain-config deprecation warning