Migrate mdn-screen.css to Stylus

The css_migration flag is added to toggle the most major effects of this
work. When the flag is enabled, mdn-screen.css is not attached to pages.
When it is disabled, mdn-screen.css is attached normally, as it was
before.

The flag should be disabled for almost all users, but it might make
sense to enable the flag for beta testers so that they can find bugs in
the migration and report on them.