Fix the signature generation for GitHub payloads.

GitHub payload signatures are encoded in SHA1, but we were encoding in
MD5. This updates our signature code and tests to encode in SHA1.

Reviewed at https://reviews.reviewboard.org/r/6321/
Fix several review fields on non-Local Sites.

Some of the earlier fixes for user and diff links on Local Sites broke
non-Local Sites. This didn't cause the review request pages to break,
but it did cause the fields to disappear. This fixes them by making use
of the new 'local_site=' parameter on local_site_reverse().

Reviewed at https://reviews.reviewboard.org/r/6320/
Merge branch 'release-2.0.x'