Perform unicode migration for reviewboard/admin
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 handle string/byte types when

communicating with memcached, some changes to use six.text_type for python 2/3

compatibility, and marking some additional strings for translation.

Ran unit tests
Smoke tested admin