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
The comment is misleading. The motd page is where we can *edit* the message. We actually include both the text in a form field and the message on the page, probably intended as a preview. But the way this is written (before this bug) the preview view is never shown. So we're not adding a close button to a message that is hidden. So yeah, lots of room for improvement there, but aside from updating the comment I'm going to leave that to another bug.