Adds `config.action_mailer.preview_enabled`

This allows mail previewing to be enabled easily in non-development
environments such as staging. The default is set to true for development
so no changes should be required to existing Rails applications.

The mail preview path can still be configured using the existing
`config.action_mailer.preview_path` configuration option.

Adding this avoids devs from having to do stuff like:
https://gist.github.com/lengarvey/fa2c9bd6cdbeba96526a

Update actionmailer/CHANGELOG with new configuration.
Update configuring guide with new configuratation.
Add `config.action_mailer.preview_path` to configuring guide.

Add punctuation and fix capitalization in Getting Started guide

[ci skip] Fix router option reference in 4.2 release notes.

Fix more small grammar mistakes in the [Getting Started guide](http://edgeguides.rubyonrails.org/getting_started.html#security), in sections 9 and 10.

Specifically, within sections 9.1 “Basic Authentication”, 9.2 “Other Security Considerations”, and 10 “What's Next?”.

“Rails Guides” is generally capitalized on the site, including on the linked page and in the title of the site, so I capitalized it.

“The” is not part of the name of the Ruby on Rails Security Guide – “The” is not in the title on that page.

And I added a missing period and a missing colon at the ends of two sentences.

[Postgresql tests] remove duplicate helper

Update the levenshtein distance method in guides.

Add missing period in Getting Started guide

Within section 5.10 Adding Some Validation

MySQL doesn't work with foreign keys

This was previously fixed in e84799d9 but broken in 3f596f8a. This commit
reintroduced the conditional that prevents the foreign keys from being
added to MySQL databases.

Fix test with `add_foreign_key` DSL

rename primary key sequence only if it exists

Always instrument ActiveSupport::Cache

Add always permitted parameters as a configurable option.

[Rafael Mendonça França + Gary S. Weaver]

The current approach is broken because it uses a thread local value
which means on multi-threaded environments it has to be turned on
per thread. Secondly, ActiveSupport::Notifications does not
instrument items when there are not subscribers so this flag is
unnecessary.

Update documentation for run_callbacks

In some cases run_callbacks will return nil.  I'm attempting to update the documentation for the method to clarify.

* General style fixes.
* Add changes to configuration guide.
* Add missing tests.

Leave the note for `ActiveModel`, since it can't yet detect mutations
(hopefully we can change this in time for 4.2). However, we now detect
mutations on all supported types in `ActiveRecord`, so we can note that
`_will_change!` is no longer needed there.

Closes #15942

Fix bug, when ':dependent => :destroy' violates foreign key constraints

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/lib/active_record/associations/builder/association.rb
	activerecord/lib/active_record/associations/builder/has_one.rb

Detect in-place modifications on Strings

Add AR::Base#valid! method

databases.rake refactoring

Stop using instance exec for type decorators

We are moving this behavior out to an object that we would like to keep
separated from `ActiveRecord::Base`, which means not passing the class
object to it. As such, we need to stop using `instance_exec`, and
instead close over the subclass on global type decorators that are
applied in `Base`.