Perform unicode migration for reviewboard/cmdline

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 fixes up some file open calls to use 'with'.

Testing done: Smoke tested rb-site and rbssh
Reviewed at https://reviews.reviewboard.org/r/4961/