Respect the @cancellable flag for DirectServe{Html,Json}Resources

`DirectServeHtmlResource` and `DirectServeJsonResource` both inherit
from `_AsyncResource`. These classes expect to be subclassed with
`_async_render_*` methods.

This commit has no effect on `JsonResource`, despite inheriting from
`_AsyncResource`. `JsonResource` has its own `_async_render` override
which will need to be updated separately.

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

synapse/http/server.py

@@ -369,6 +369,8 @@ async def _async_render(self, request: SynapseRequest) -> Optional[Tuple[int, An
 
         method_handler = getattr(self, "_async_render_%s" % (request_method,), None)
         if method_handler:
+            request.is_render_cancellable = is_method_cancellable(method_handler)
+
             raw_callback_return = method_handler(request)
 
             # Is it synchronous? We'll allow this for now.