Fix deactivation running off the main process (#18716)

Best reviewed commit by commit.

With the new dedicated MAS API
(https://github.com/element-hq/synapse/pull/18520), it's possible that
deactivation starts off the main process, which was not possible because
of a few calls.

I basically looked at everything that the deactivation handler was
doing, reviewed whether it could run on workers or not, and find a
workaround when possible

---------

Co-authored-by: Eric Eastwood <erice@element.io>

docker/configure_workers_and_start.py

@@ -178,6 +178,7 @@ WORKERS_CONFIG: Dict[str, Dict[str, Any]] = {
             "^/_matrix/client/(api/v1|r0|v3|unstable)/login$",
             "^/_matrix/client/(api/v1|r0|v3|unstable)/account/3pid$",
             "^/_matrix/client/(api/v1|r0|v3|unstable)/account/whoami$",
+            "^/_matrix/client/(api/v1|r0|v3|unstable)/account/deactivate$",
             "^/_matrix/client/(api/v1|r0|v3|unstable)/devices(/|$)",
             "^/_matrix/client/(r0|v3)/delete_devices$",
             "^/_matrix/client/versions$",