Make the match email signature be data-driven

[rjsparks]

Right now, when the guides program leadership changes, someone has to manually edit the footer at guides/templates/guides/match_email.txt (example: fddeeac). The User class should be extended to an indication for whether a name should be added to this list, perhaps with an ordering, so that the change can be made in the data.

Alternatively, a string, editable through the admin, could be stored in the database and used by this template (similar to a snippet).