Add a public API to allow users to specify column types

As a result of all of the refactoring that's been done, it's now
possible for us to define a public API to allow users to specify
behavior. This is an initial implementation so that I can work off of it
in smaller pieces for additional features/refactorings.

The current behavior will continue to stay the same, though I'd like to
refactor towards the automatic schema detection being built off of this
API, and add the ability to opt out of automatic schema detection.

Use cases:

- We can deprecate a lot of the edge cases around types, now that there
  is an alternate path for users who wish to maintain the same behavior.
- I intend to refactor serialized columns to be built on top of this
  API.
- Gem and library maintainers are able to interact with `ActiveRecord`
  at a slightly lower level in a more stable way.
- Interesting ability to reverse the work flow of adding to the schema.
  Model can become the single source of truth for the structure. We can
  compare that to what the database says the schema is, diff them, and
  generate a migration.
Add description of ActionMailer::TestHelper module [ci skip]
Use "Action Mailer" as canonical name in documentation [ci skip]

See also:
https://github.com/rails/rails/commit/3a297f8db75b172882bb3c2f9440d0c7365b8b11#commitcomment-6451708
Remove unnecessary branch from quoting in Mysql

This is already the behavior for `BigDecimal` in the abstract adapter.
Pass through AM documentation re: 02d048d [ci skip]
Merge pull request #15284 from sgrif/sg-properties

Add a public API to allow users to specify column types
Typo in AR CHANGELOG [ci skip]
Merge pull request #15353 from sgrif/sg-mysql-condition

Remove unnecessary branch from quoting in Mysql
implement ActiveRecord::Base#pretty_print + changelog