Add `MailDeliveryJob` for delivering both regular and parameterized mail.
Deprecate using `DeliveryJob` and `Parameterized::DeliveryJob`.

* Eager autoload mail gem when eager load is true

We had a production issue where our Sidekiq worker threads all became
deadlocked while autoloading a file within the mail gem, required via
ActionMailer, despite setting our Rails applicaiton to eager load.
`Mail.eager_autoload!` exists and works great, ActionMailer just doesn't
call it during eager loading. Adding it to the ActionMailer Railtie's
eager_load_namespaces takes care of calling `Mail.eager_autoload!`
during the `eager_load!` initializer.

* 'Mail' isn't defined yet, use before_eager_load instead

* Make sure mail is loaded

* Move eager load of Mail into ActionMailer.eager_load!

[Samuel Cochran + Rafael Mendonça França]

This basically reverts cd9cc721

This reverts commit 3420a145, reversing
changes made to afb66a5a.

Without this, Action Mailer doesn't work like it used to on version 4.
The following snippet fails since version 5:

    > require 'action_mailer'
    > ActionMailer::Base
    NameError: uninitialized constant ActiveSupport::Rescuable

Offer the option to use parameterization for shared processing of headers and ivars

Per https://www.timeanddate.com/counters/firstnewyear.html, it's already
2017 in a lot of places, so we should bump the Rails license years to
2017.

[ci skip]

The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

Abstract Controller is the common component between Action Mailer and
Action Controller so if we need to share the caching component it need
to be there.

When used by ActionMailer, ActionView should automatically use the correct MIME type just as it does when used by ActionDispatch #11157

Explain what the interceptor is used for and how to remove it.

The new option allows any Ruby namespace to be registered and set
up for eager load. We are effectively exposing the structure existing
in Rails since v3.0 for all developers in order to make their applications
thread-safe and CoW friendly.

This is a private place to put those AS features that are used
by every component. Nowadays we cherry-pick individual files
wherever they are used, but that it is not worth the effort
for stuff that is going to be loaded for sure sooner or later,
like blank?, autoload, concern, etc.

Signed-off-by: NSantiago Pastorino <santiago@wyeworks.com>