Support MSC3916 by adding a federation /thumbnail endpoint and authenticated `_matrix/client/v1/media/thumbnail` endpoint (#17388)

[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

1 files changed, 0 insertions, 4 deletions

diff --git a/synapse/config/experimental.py b/synapse/config/experimental.py

index 1b72727b75..c21b7eb37e 100644
--- a/synapse/config/experimental.py
+++ b/synapse/config/experimental.py
@@ -437,10 +437,6 @@ class ExperimentalConfig(Config):
             "msc3823_account_suspension", False
         )
 
-        self.msc3916_authenticated_media_enabled = experimental.get(
-            "msc3916_authenticated_media_enabled", False
-        )
-
         # MSC4151: Report room API (Client-Server API)
         self.msc4151_enabled: bool = experimental.get("msc4151_enabled", False)