Perform unicode migration for reviewboard/accounts

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 string-related change to use six.text_type for python 2/3
compatibility.

Testing done:
* Ran unit tests
* Smoke tested login/logout/settings

Reviewed at https://reviews.reviewboard.org/r/4957/