Add support for more headers on legacy emails

Problem/Motivation

The legacy email builder only handles body and subject. It would be useful to include more headers, especially addresses.

Proposed resolution

Create two new helper functions on MailManagerReplacement::emailFromArray() and ::emailToArray(). Call these in LegacyEmailBuilder and MailManagerReplacement::mail().

Remaining tasks

User interface changes

API changes

Not BC: Changes the definition of hook_mailer_bc_alter(). The new definition feels much better because it passes a message array in the well-known format.

Data model changes