Perform unicode migration for reviewboard/webapi

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 a number of changes to use six.text_type for python 2/3
compatibility, using 'with' for file opens, as well as some clean-ups to use
raw strings when testing escaping.

Testing done: Ran unit tests

Reviewed at https://reviews.reviewboard.org/r/4975/