Fix some harmless PyFlakes warnings.

A couple of these looked like errors to PyFlakes, but just because of
our control flow. I've made PyFlakes happy again.

Mostly, this is removing unused imports, and adding some default values.
Release Review Board 2.0 beta 3.
Fix the Djblets dependency, and re-release 2.0 beta 3.

The Djblets dependency had the wrong version. It's now fixed.
Bump to 2.0 beta 4 dev.

This will likely become RC, barring any major bugs in beta 3.
Fix DifFParser preamble parsing with weird unicode.

One of the old review requests on r.rb.o has a diff file which is basically
utf-8 encoded garbage. This was causing an exception because we were trying to
concatenate strings and bytes together, which caused it to try to decode the
utf-8 text as if it were ascii. Adding some 'b' prefixes to our strings keeps
things in the bytes realm.

Testing done:
Looked at the /diff/raw/ page for both the broken review request, as well as a
couple ones that had valid diffs.