Handle unicode/str cast operators.
A few of our objects defined __str__ and/or __unicode__ methods. This presents

some complications for porting to python 3, since str has different meanings.
Django provides a decorator, @python_2_unicode_compatible, whereby you define

__str__ using python 3 semantics (i.e. it returns unicode), and when running

with python 2, it will rename that to __unicode__ and make __str__ call that

and encode to utf-8.

Ran djblets unit tests

Ran Review Board unit tests