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
ah, we're just using `sessionStorage` elsewhere. it's fine then, since this function gets called only once.