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

[matthazinski]

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.

[pirate]

Note that our DB latency is almost nothing as we use SQLite which serves from memory + filesystem, so adding cache for DB queries (e.g. from_json) is not worth it. But cache is still useful to avoid heavy filesystem operations like get_dir_size.

If you want to open a PR for that I'd review it!

I don't want to add redis by default / push people towards redis, it's too much complexity for now, but happy to add a filesystem cache, computed db field, or in-memory cache that doesn't require additional config to setup.