Switch to using ForwardingCacheBackend for all cache requests.

We now force ForwardingCacheBackend as the default cache backend, and
instead work with the 'forwarded_backend' cache name when looking up or
saving the real cache backend.

This covers the configuration forms, cache stats, and initial settings
population.

Reviewed at https://reviews.reviewboard.org/r/6482/
Fix "List index out of range" errors with interdiffing deleted files.

It's possible to hit an IndexError when processing an interdiff between two
changes that delete a file. The wrinkle here is that between the two revisions
of the diff, some additional changes to that file were made (and synced to the
local developer's tree).

The result of this is that we didn't filter out the file from the list of
things to show for the interdiff, and then things went wrong when we tried to
process the opcodes for diffing two empty strings.

While I was doing this, I saw some warnings about a unicode-unaware comparison
occurring, so I've slipped the fix for that in as well. This doesn't actually
have any correctness implications, it just speeds up things slightly when
dealing with empty files.

Testing done:
- Created a reproduction case for the bug and tested before and after the
  change. Before, I saw an entry for this file with the error inside. After,
  the deleted files don't show up at all in the interdiff.
- Ran unit tests.

Fixes bug 3481.

Reviewed at https://reviews.reviewboard.org/r/6473/