[MSC3916](https://github.com/matrix-org/matrix-spec-proposals/pull/3916) added the endpoints `_matrix/federation/v1/media/thumbnail` and the authenticated `_matrix/client/v1/media/thumbnail`. This PR implements those endpoints, along with stabilizing `_matrix/client/v1/media/config` and `_matrix/client/v1/media/preview_url`. Complement tests are at https://github.com/matrix-org/complement/pull/728