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 do want to fix things right and not just close bugs, but I'm also concerned about scope creep. I would much prefer to fix this bug as specified (more or less, it didn't make sense to me to go to /reviewers/apps/motd to set the message for developers, so I put it under developers/motd even though that was more work) and have a discussion about what the right UI is. I get the feeling that a lot of folks have problems with our current menu structure, or lack thereof, and that it is being actively worked on. Let's have that discussion, get the design docs, and implement the right thing for the whole site, rather than changing things by fiat for this one thing.