Email footer support

[downey (Michael Downey)]

See also this related feature request:

Subscription emails should carry more context in message body

Sometimes when I receive notification emails, I can’t remember why. Even though the context is all in the message subject, some additional info would be nice. Possibly add to the message footer, after the “To respond…” line, something like: You received this message because you are subscribed to email notifications for the "XYZ" category at Meta Discourse.