Create a Mail plug-in that emulates swiftmailer

Problem/Motivation

Given that swiftmailer is unsupported, then sites need a drop-in replacement that is the minimum possible change.

Proposed resolution

• Rename sub-module "Symfony Mailer Back-compatibility" to "Symfony Mailer Replace"
• Move LegacyEmailBuilder into the symfony_mailer module - it will be used in both "replace" and "plug-in" modes of operation.
• Create a new service LegacyMailerHelper. Move in the code from LegacyEmailBuilder::emailFromArray(), splitting out the part for the body into a separate function formatBody(). Create another function mail() taking code from MailManagerReplacement::mail().
• Create a plug-in class SymfonyMailer of type @Mail which maps directly onto functions from LegacyEmailBuilder.

Equivalence of swiftmailer code:

• Code relating to swiftmailer.transport config is taken over by Symfony Mailer transport configuration. SwiftmailerEmailBuilder contains an import function.
• Code relating to swiftmailer.message config is mostly handled by other settings. Content type maps to the new adjuster from #3264279: Send email as plaintext → .
• Character set (if still relevant today) would map to a new adjuster. Symfony Mailer library will always generate a plain text so that's redundant. Text format (if still needed) would likely need to be a new config setting.
• Code relating to attachments is mostly handled by Symfony Mailer automatically.
• Hooks: We can't support hook_swiftmailer_alter() as it takes the swiftmailer specific class. We could probably support hook_swiftmailer_attach(), it could be an EmailProcessor.
• Library: We could support the swiftmailer library - this could be an EmailProcessor.

Remaining tasks

User interface changes

API changes

Data model changes