Emails: add post type and taxonomy, and supporting wp-admin customisations.

All of BuddyPress' emails have been moved into email posts. The change includes an installation routine, but this won't be run until the db_version number is bumped in a subsequent commit. Updates to functions using wp_mail will also follow.

Tokens are used to personalise the email content (e.g. to add a link to the recipient’s user profile). Each type of email in BuddyPress has been assigned a unique type, and these are mapped to the email post through a new "email type" taxonomy.

The change includes a new HTML email template based on work by Ted Goas and contributors from the Cerberus email templates project. Ted, thank you very much. Learn more at ​http://tedgoas.github.io/Cerberus/

See #6592. Props timersys, mercime, boonebgorges, hnla, DJPaul.