This change fixes up a handful of issues related to string types. Some

of these are more important than others.
For a couple places, we had code deep in the diff rendering which was

creating a SafeString. Because we have caching at many levels in the

diff viewer, these values would end up trying to go through pickle to

store the rendered data. When pickle encounters a value that it doesn't

understand, it tries to call str() on it. With the older Django

version, this would correctly convert, but with the modern Django,

calling str() on a SafeString would return the SafeString again.

This would just end up infinitely recursing inside of the pickle

machinery. There are two places where this affects. For the first, the

fix is to remove some old mark_safe calls. For the second (where we

were rendering a template), the solution is to explicitly convert to

str by using a slice trick.
Next, the HTTP digest authentication backend was calling MD5, but

passing in an str instead of bytes. We also had a handful of other

places where there was some confusion about bytes/str which was found

via static analysis.
Finally, a bunch of unit tests were passing in int types for the

revision when creating test filediffs. These do end up getting cast to

str, but since we say the type of the argument is str, it's more correct

to pass that in directly.

Created a review request with a diff and was able to successfully load

  the diff viewer page.
Ran unit tests.