When a module rejects a piece of media we end up trying to close the same logging context twice. Instead of fixing the existing code we refactor to use an async context manager, which is easier to write correctly.
[MSC3916](https://github.com/matrix-org/matrix-spec-proposals/blob/rav/authentication-for-media/proposals/3916-authentication-for-media.md) adds new media endpoints under `_matrix/client`. This PR adds the `/preview_url`, `/config`, and `/thumbnail` endpoints. `/download` will be added in a follow-up PR once the work for the federation `/download` endpoint is complete (see https://github.com/element-hq/synapse/pull/17172). Should be reviewable commit-by-commit.