Automatically transition away from older memcached backends.

Historically, the memcached backend used the class name 'CacheClass',
which we've been setting and many people have stored. While many
backends in Django do still have their CacheClass equivalent, memcached
does not.

We now fix up the name when normalizing the cache backend, which happens
on site settings load. This guarantees nobody will break.

Reviewed at https://reviews.reviewboard.org/r/6481/