Backing up and restoring of email history

[philcali]

This feature allows for users to optionally back up email history with the appropriate courseid and userid mapping on restore.

Some important notes:

• On backup, a user can optionally include email history if users, role assignments, and blocks are backed up
• On restore, a user can only restore if users, role assignments, and blocks are present in the backup
• On an overwriting restore, a user can optionally clear out the current course email history (defaults to no).
• If one can overwrite, then they can only do so if restoring the history.

Sounds complicated, but it's pretty natural when clicking on the backup and restore options.

Currently, alternate email ids and drafts are unsupported.