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
Is it typical to make localization keys unicode? On the other hand, the get_config and set_config should probably both be unicode or both be strings. I don't actually see a reason for them to be unicode.