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'.

Ran unit tests.