mimemail 8.x-1.0-alpha4

Changes since 8.x-1.0-alpha3:

• #3213974 by TR: Remove deprecated drupalPostForm from functional tests
• #3164459 by TR: Wrong type in a @var comment causes problems with static analysis
• #3164457 by TR: Don't mask $this->configFactory in MimeMail mail plugin
• #3164453 by TR: Restore Rules as a test_dependency
• #3155729 by TR: Duplicate file attachments are not prevented
• #3155337 by TR: MimeMailFormatHelper::testUrl() use assertSame() with $expected and $actual in the wrong order
• #3155011 by TR: Respect $message langcode when filtering HTML email
• Some documentation comment clean-up.
• #3154658 by TR, rivimey: Pull all static \Drupal:: calls to top of functions for clarity and performance
• #3153609 by TR: Per-user plaintext preference was never ported to D8
• #3153695 by TR: Stop using PHP superglobals
• #3153600 by TR: Inline the preg_replace_callback functions
• #3153578 by TR, rivimey: Use CRLF as line terminator in message body
• #3126782 by TR: Replace file_uri_scheme() when Drupal 8.8 becomes the lowest-supported core version
• #3147165 by TR: Declare explicit dependency on D8.8+ / Drop support for D8.7