We do a few things in this PR to better support caching:
1. Change `Cache-Control` header to allow intermediary proxies to cache
media *only* if they revalidate on every request. This means that the
intermediary cache will still send the request to Synapse but with a
`If-None-Match` header, at which point Synapse can check auth and
respond with a 304 and empty content.
2. Add `ETag` response header to all media responses. We hardcode this
to `1` since all media is immutable (beyond being deleted).
3. Check for `If-None-Match` header (after checking for auth), and if it
matches then respond with a 304 and empty body.
---------
Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>
I thought ruff check would also format, but it doesn't.
This runs ruff format in CI and dev scripts. The first commit is just a
run of `ruff format .` in the root directory.
