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
If we had a bunch of these, event delegation would make sense, but this is effectively a one-off so we might as well bind it where we find it.