Ensure CDN invalidations occur when crates or crate versions are deleted

[LawnGnome]

This can be reviewed commit-by-commit if you want, but it's probably small enough to digest without doing so.

Basically: we need to invalidate the CDNs when crate files (and related artifacts) are deleted, otherwise a crate that reuses the same name might still have the deleted crate file served from a CDN node.

I initially debated implementing this inline in the two different places we'd need it via a new Environment::invalidate_cdns_many function, but it felt cleaner to implement it as a new background job (and it doesn't feel like it's super time critical, most of the time), so here we are.

I have one mild concern about this: namely, because Fastly requires one API request per path to be purged — and we have to purge two paths for each crate version, plus the RSS feed if we're deleting the whole crate — we may go somewhat closer to hitting the rate limit than previously. But I think let's try it and see what happens in practice.

I'd like to give this a test on staging before we deploy it, ideally.

Fixes #10887.

[Turbo87]

LGTM at first glance

[LawnGnome]

Tested on staging; seems to work as expected, both for version and full crate deletions.

Should be good to deploy whenever we next deploy.