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'
writing the new body can cause the response to be committed and the
request thread to return up the stack before the instance variable is
assigned.  Synchronize so that the ivar is assigned before the other
thread can activate.

fixes #10984
use bytesize rather than force encoding
remove deprecated implicit join references.
fetch value(s) from stringified options
Update a broken link into the fixtures' template
Merge pull request #11175 from robin850/patch-4

Update a broken link into the fixtures' template [ci skip]
Merge pull request #11152 from senny/remove_implicit_join_references

remove deprecated implicit join references
provide a more sementicthe local variables name for ActiveModel::Validations::Clusivity#include? method
the original name `exclusion` is a bit confusing when using with the method `inclusion_method`
rename it to a more logic neutral name.
Merge pull request #11156 from JonRowe/fix_tags_picking_string_keys_from_non_stringified_options

Fetch value(s) from stringified options in tags that stringify options
Merge pull request #11177 from yangchenyun/mine

provide a more sementic local variables name in ActiveModel::Validation::Clusivity
Fix `rails plugin --help`

Right now if you run the `rails plugin --help` command it fails because rails expects a command in `railties/lib/rails/commands/plugin.rb` that does not exist because the file is named `plugin_new`. This is the error:

```
ruby-2.0.0-p0  ~/documents/projects/tmp/vanilla (master)
$ rails plugin --help
/Users/schneems/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in `require': cannot load such file -- rails/commands/plugin (LoadError)
	from /Users/schneems/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in `block in require'
	from /Users/schneems/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:213:in `load_dependency'
	from /Users/schneems/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in `require'
	from /Users/schneems/.rvm/gems/ruby-2.0.0-p0/gems/railties-4.0.0/lib/rails/commands.rb:49:in `<top (required)>'
	from bin/rails:4:in `require'
	from bin/rails:4:in `<main>'
```
cookies are encrypted starting Rails 4
Merge pull request #11176 from schneems/schneems/plugin_new_help_fix

Fix `rails plugin --help`
doc for more input tag types
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]