* 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]