Perform unicode migration for reviewboard/*.py

This change represents an audit of all of our string literals to make sure that
bytestrings are annotated with a leading 'b', and then adds
"from __future__ import unicode_literals" to the beginning of every non-empty
file. I've included some pretty simple cleanups for a couple trivial issues as
well.

Testing done: Ran unit tests

Reviewed at https://reviews.reviewboard.org/r/4976/