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'.

Ran unit tests
Smoke tested the diff viewer