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
Even so, I still don't think it makes a whole lot of sense. It seems more intuitive to just have a link to "Edit" to something that can be edited. Until I click it, it isn't clear that "Announcement" means "Edit Message of the Day."