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 agree with your concern for scope creep for bugs. In this case, you're not _changing_ an existing link, you're instead adding a new link (albeit in a style that is consistent with the style of an existing link elsewhere: the Reviewer Tools).


As a rule of thumb, new features should never make the UX worse or more confusing. It is part of the review process to take this feedback and realise opportunities to improve __our__ code (semantically, functionally, and visually).

We could wait for a redesign to solve all these problems at once. Or we could solve each problem incrementally as it comes up. It's easier to achieve polish when you fix things incrementally and take care of them now. And with a redesign, at least when it is first introduced, it is actually harder to achieve polish.

Papercuts happen when new issues or complexities are introduced and we say we will fix them _later_. I want to close a bug just as much as the next dev, but it pays to take the extra time to be diligent, pay close attention to the details, accept review feedback, and not save things for some other time or some other dev.