Remove a bunch of the review-related globals.

We used to have a bunch of globals for various bits of state, like the
review request URL, whether or not we can edit, the bug tracker URL,
etc. These have been removed, in favor of attributes on the new objects.
Most of these weren't even in use anymore.

Played around with the UI a bit and tested the affected code paths. Didn't
see any problems.

Unit tests all pass.