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
if you do that, you can remove the intermediate variable. it's really that I like things related things to happen in the same place. so I'd recommend at the very least moving `closeButton.click` up to line 1351. no need to do that below the storage check.