Add support to attachments for LegacyEmailBuilder

Problem/Motivation

Legacy emails commonly list attachments in $params['attachments']. We can support these by converting to Symfony Mailer attachments.

Proposed resolution

Roughly based on SwiftMailer::mail().

Remaining tasks

1. Some code - possibly older - uses $params['files'] and $params['images']. We won't implement this at first, however
2. The implementation uses the default Symfony MIME type guesser which is potentially slightly different from the Drupal one. If needs be we could later change this.

User interface changes

API changes

Data model changes