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.

Testing done:
* Ran unit tests
* Smoke tested admin

Reviewed at https://reviews.reviewboard.org/r/4958/