Audit all strings in djblets and convert to __future__.unicode_literals

This change represents an audit of all of the strings in the djblets codebase.
Where the strings were actually binary data (specifically with our treatment of
settings.EMAIL_HOST_USER and settings.EMAIL_HOST_PASSWORD, and anything passed
into sha1() or md5()), I've made it use the 'bytes' type. Otherwise, these are
now unicode objects by virtue of importing unicode_literals from __future__.

There's also a bunch of fixes to our handling of cache keys, especially those
for large data, much more consistent. We always keep the cache key as a unicode
object and create derivative keys (like the chunk keys for large data) with
that. At the last moment, before calling cache.get() or cache.set(), we pass
that to make_cache_key, which will shorten it if necessary using md5, and then
encode to utf-8 (since not all cache backends support the unicode object as
keys).

Testing done:
* Verified my assumptions about treatment of unicode vs. str in getattr,
  hasattr, delattr, __getattr__, __hasattr__, __delattr__, hash, regular
  expressions, and memcache keys.
* Ran djblets unit tests
* Ran Review Board unit tests

Reviewed at https://reviews.reviewboard.org/r/4944/