Refactor logic out of UploadDiffForm

All of the logic for taking a diff file and creating a DiffSet was in
UploadDiffForm. This change moves it out into a manager, so that I'll be able to
use the same code for creating DiffSets from diff data fetched from an upstream
repository.

Testing done: ran unit tests.
Reviewed at http://reviews.reviewboard.org/r/4231/
Move the data.expand setting into syncOptions.

This ensures that data.expand is set even if other form data is set.
This is needed for POST requests.
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.