Add developer MOTD (bug 945956)

Added MOTD to My Submissions page, along with page for editing it, permission for editing. Moved some CSS and JavaScript around to make existing MOTD code more general.

Squashed commit messages:
needs ACL
implemented motd for My Submissions page
added menu to set developer motd
added migration
https://github.com/mozilla/zamboni/blob/master/mkt/api/tests/test_forms.py for example. Personally I didn't comment on this because this is a pretty straightforward form with only required field that shouldn't really need testing. The win isn't worth it IMHO.