Cache `get_dir_size()` to avoid slow list rendering performance on `/admin/core/snapshot/`

I noticed listing snapshots is really slow. I found a few optimizations that seem to increase performance, but wanted to know whether there any undesirable side effects to this:

- `get_dir_size()` requires recursing the snapshot directories for every snapshot on a page. It seems `archive_size()` caches this, but only for the default Django cache TTL (300 seconds). Is there any reason we can't just set `CACHES['default']['TIMEOUT'] = None` to ensure these keys don't expire by default?
- Archivebox doesn't expose any options for choosing an external cache, which isn't great when running in ephemeral containers. I've had luck with configuring `django_redis` in `settings.py` so the cache can be periodically written to disk. If there's interest, I can put up a PR which optionally enables this. (Alternatively, if there are no blockers to upgrading to Django>=4.0, we can use the built-in Redis cache client.)
- With a warm cache, `Snapshot.from_json()` requires a round trip to both the DB and the cache. Calling `self.tags_str()` with `nocache=False` seems to cut-down DB latency by about half according to the Django debug toolbar.

Start at /admin/core/snapshot/ and trace get_dir_size(), archive_size(), Snapshot.from_json(), and tags_str(), then inspect settings.py cache configuration. Compare cold and warm cache rendering and database/cache calls; done means a decided cache policy and measured faster listing without stale or unavailable-cache failures.

Written by the indexing model from the issue text.