Perform unicode migration for reviewboard/diffviewer

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.

This also includes some changes to use six.text_type for python 2/3
compatibility and updates file opens to use 'with'.

Testing done:
* Ran unit tests
* Smoke tested the diff viewer

Reviewed at https://reviews.reviewboard.org/r/4962/