Add docs for `first` when provided a numerical argument. 

Since `first!` behaves exactly the same way but can raise an argument we can consolidate it in the `first` section.

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`.

It allows the code to be more declarative and elegant.

`ActiveRecord::FixtureSet.create_fixtures` can accept an array of
fixture_files.