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
When I uncomment this I get "assert addon_id or app_slug, 'Must provide addon_id or app_slug'", but not sure what decorator to use here instead.