Setting delivery_job on a mailer class will cause MessageDelivery to use
the specified job instead of ActionMailer::DeliveryJob:

    class MyMailer < ApplicationMailer
      self.delivery_job = MyCustomDeliveryJob

      ...
    end

[ci skip]

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

Follows the same pattern as controllers and jobs. Exceptions raised in
delivery jobs (enqueued by `#deliver_later`) are also delegated to the
mailer's rescue_from handlers, so you can handle the DeserializationError
raised by delivery jobs:

```ruby
class MyMailer < ApplicationMailer
  rescue_from ActiveJob::DeserializationError do
    …
  end
```

ActiveSupport::Rescuable polish:
* Add the `rescue_with_handler` class method so exceptions may be
  handled at the class level without requiring an instance.
* Rationalize `exception.cause` handling. If no handler matches the
  exception, fall back to the handler that matches its cause.
* Handle exceptions raised elsewhere. Pass `object: …` to execute
  the `rescue_from` handler (e.g. a method call or a block to
  instance_exec) against a different object. Defaults to `self`.

- Fixed statement about setting `config.action_mailer.default_url_options = {protocol: 'https'}` . We are just setting the protocol key to 'https', not replacing/initializing the complete config.
- Fixed grammar in assert_emails changlog
- Added sentence separator for code ":"

[ci skip]

They would be lost when the delivery job is enqueued, otherwise.
Prevents a common, hard-to-find bug like:

```ruby
message = Notifier.welcome(user, foo)
message.message_id = my_generated_message_id
message.deliver_later
```

The message_id is silently lost here! *Only the mailer arguments are
passed to the delivery job.*

This raises an exception now.

Make modifications to the message within the mailer method or use a
custom Active Job to manage delivery instead of using #deliver_later.

Removes `-t`

Adds changelog headers for beta3 release

Whenever you are sending emails in integration tests using the `:test`
delivery method you need to make sure that
`ActionMailer::Base.deliveries` is reset after every test. This piece of
boilerplate code is present in all my applications that send
emails. Let's have `ActionDispatch::IntegrationTest` reset the
deliveries automatically.

[ci skip]

During the `5.0.0.beta1` release, the CHANGELOGs got an entry like the
following:

```
* No changes.
```

It is kinda confusing as there are indeed changes after it. Not a
biggie, just a small pass over the CHANGELOGs.

[ci skip]

This reverts commit 65a61ab7, reversing
changes made to 14314ca1.

This PR broke the build

`config.force_ssl = true` will set
config.action_mailer.default_url_options = { protocol: 'https' }

If you have turned on force_ssl, and then gone to the effort of setting
config.action_mailer.default_url_options = {host: 'example.com'} then
you are probably pointing people back to your current app and want
https on that too.

This reverts commit f2a8c236, reversing
changes made to 3046c9bb.

When #deliver_now is called all translations within the
generated email will be looked up for the current I18n
locale.

    I18n.locale = ‘de’
    mail.deliver_now # Generates german email, correct

In #enqueue_delivery the locale was not considered and
the resulting job uses the default locale.

    I18n.locale = ‘de’
    mail.deliver_later # Generate english email, incorrect

In order to achieve a consistent behaviour the current locale
is now always passed to `ActionMailer::DeliveryJob`.

Also add a CHANGELOG entry for #18587

[ci skip]

Use a preview interceptor to search for inline cid: urls in src
attributes and convert them to data urls.

Mailer preview now uses `url_for` to fix links to emails for apps running on a subdirectory, closes #19092.

While this was true before when every `dd` had a value,
this patch makes sure that everything keeps lining up even
when the `dd` node is blank.

One or multiple mail interceptors can be unregistered using
`ActionMailer::Base.unregister_interceptors` or
`ActionMailer::Base.unregister_interceptor`.

For preview interceptors, it's possible to use
`ActionMailer::Base.unregister_preview_interceptors` or
`ActionMailer::Base.unregister_preview_interceptor`.

Refactors logic to constantize a string/symbol into separate method.

Following the same naming convention used in
controllers and jobs.