Django 4.2 introduced a new way of configuring and accessing storage

backends. Rather than separate implementations for file storage and

static media storage, everything now goes through a STORAGES

backend dictionary and a storages instance dictionary.
We now configure these in our Djblets-provided settings and our test

settings, and update the dictionary for extension packaging. The

extension packaging will need further tests, as storage handler caches

can't reasonably be cleared (without a hack to clear cached properties),

but the old storage support appears to have also cached state. So it

should be fine, but testing will be needed as development continues.
The avatars tests has been updated to use the storages instance

dictionary instead of the deprecated get_storage_class() method.
This removes all storage-related warnings from the test storage-related

warnings from the test suite.

Unit tests pass.