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
Not hidden in developer tools, but hidden in reviewer tools. https://github.com/mozilla/zamboni/blob/master/media/css/zamboni/editors.styl#L352 I'm trying to keep the same function working for both, because we already have enough duplication because we already have enough duplication because we already have enuogh duplication.