Improve back-compatibility, call hook_mail_alter()

Problem/Motivation

Sites migrating from swiftmailer need a solution that's as safe as possible, minimizing the required changes.

Proposed resolution

Sites that want the minimum changes should enable symfony_mailer is enabled but not symfony_mailer_bc. This will be quite similar to swiftmailer, although still some differences that could be fixed as follow on issues:

• The name and variables for templates have changed.
• The name of the CSS library for mail CSS has changed.

Details

1. Move MailManagerReplacement, LegacyEmailBuilder and symfony_mailer_bc_mailer_builder_info_alter() into the main module.
2. LegacyEmailBuilder calls hook_mail_alter()

Remaining tasks

User interface changes

API changes

Data model changes