Fixed #35514 -- Implemented dictionary-based MAILERS.

See DEP 0018.

Added:
* MAILERS setting.
* django.core.mail.mailers dict-like EmailBackend factory.
* `using` argument to mail sending APIs.
* `sent_using` attribute to mail.outbox messages in locmem backend.
* MAILERS in startproject settings template, set to console backend.
* AdminLogHandler.using argument.
* BrokenLinkEmailsMiddleware.send_mail() method.

Updated:
* BaseEmailBackend to track the MAILERS alias used to construct it, and
  to report errors for unknown kwargs (OPTIONS).
* EmailBackend implementations to initialize from kwargs (OPTIONS) only
  when MAILERS is being used.
* smtp.EmailBackend to require `host` option and to default `port`
  option based on SSL/TLS options.
* SimpleTestCase setup to substitute the locmem backend for all defined
  MAILERS configurations.
* Django's tests that send mail to define MAILERS.

Deprecated:
* EMAIL_BACKEND and other backend-related EMAIL_* settings.
* mail.get_connection().
* The `connection`, `fail_silently`, `auth_user`, and `auth_password`
  arguments to mail functions.
* The EmailMessage.connection attribute.
* BaseEmailBackend support for `fail_silently`. Backends that support
  fail_silently (SMTP, console, file) now implement it directly.
* AdminEmailHandler.email_backend argument.

Removed undocumented features without deprecation:
* EmailMessage.get_connection() method. (send() now raises an error if a
  subclass has attempted to override it.)
* EmailMessage.send() no longer sets self.connection to the connection
  used for sending. (It still _uses_ a pre-existing self.connection.)
* AdminEmailHandler.connection() method. (Init now raises an error if a
  subclass has attempted to override it.)

Thanks to Natalia Bidart for shepherding DEP 0018 and for extensive
reviews and suggestions on the implementation.

Thanks to Jacob Rief for the initial implementation and multiple
iterations while refining the design.

Co-authored-by: Jacob Rief <jacob.rief@gmail.com>

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