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'
Remove a duplicated section [ci skip]

Attachments were previously covered so remove the useless part. Just
move a note about multipart headers set when the mail method is
triggered to the kept section.
Added clarification for destroy link_to description in 5.14 [ci skip]
Typo in Active Record Migrations Guide [ci skip]
store a symbol directly inside DISPATCH_CACHE for extra roflscale
Clean the code a bit
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)`
updated AS:Callbacks doc for terminator option in define_callbacks method
The change is commited at ba552764344bc0a3c25b8576ec11f127ceaa16da
Merge pull request #11190 from dkaplan88/typo

Typo in Active Record Migrations Guide
Merge pull request #11189 from jetthoughts/destroy_link_description_for_getting_started

Added clarification for destroy link_to description in 5.14
Merge pull request #11180 from robin850/patch-5

Remove a duplicated section [ci skip]
Synchronize PostController#create code from 5.6 to others sections. [ci skip]

Fixed `permit` using for comments.
remove evals from AM::Validations::Callbacks
follow the same refactor at a63a964a5d1ed02cf0df1b1a33a96ed2a9fa987b
don't shadow `through_scope` method name with local var.
Merge pull request #11186 from jetthoughts/synchronize_create_method_body

Synchronize PostController#create code from 5.6 to others sections in Getting Started guide.
Removed params permit step from 5.6 section [ci skip]
Merge pull request #11196 from yangchenyun/remove_evals_in_am_callbacks

Remove evals from AM::Validations::Callbacks
fix typo in ActiveModel::Error docs [ci skip]