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
I think this is perfectly well within the scope of this bug. Simply, instead of adding a new link to the nav, you'd just add a link below the MOTD. Bugs don't exist just to be closed. When bugs are resolved fixed, their solutions should be ideal ones and not introduce new issues or confusions.

If you don't want to do this as part of the bug, that's fine. But it will come up and someone else will remark about what I just addressed.