Perform unicode migration for reviewboard/reviews

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 changes some file opens to use 'with'.

Testing done: Ran unit tests

Reviewed at https://reviews.reviewboard.org/r/4966/