Respect the `@cancellable` flag for `ReplicationEndpoint`s (#12700)

While `ReplicationEndpoint`s register themselves via `JsonResource`,
they pass a method that calls the handler, instead of the handler itself,
to `register_paths`. As a result, `JsonResource` will not correctly pick
up the `@cancellable` flag and we have to apply it ourselves.

Signed-off-by: Sean Quah <seanq@element.io>