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.

Ran unit tests
Smoke tested login/logout/settings