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
That doesn't even belong to this bug. I installed the flake8 plugin for sublime text and thought I could fix things as I went, but that will definitely have to wait for another day and another bug. Reverting that line.