Restricted join support on /make_join, /send_join (#2478)

* Add `QueryRestrictedJoinAllowed`

* Add `Resident` flag to `QueryRestrictedJoinAllowedResponse`

* Check restricted joins on federation API

* Return `Restricted` to determine if the room was restricted or not

* Populate `AuthorisedVia` properly

* Sign the event on `/send_join`, return it in the `/send_join` response in the `"event"` key

* Kick back joins with invalid authorising user IDs, use event from `"event"` key if returned in `RespSendJoin`

* Use invite helper in `QueryRestrictedJoinAllowed`

* Only use users with the power to invite, change error bubbling a bit

* Placate the almighty linter

One day I will nuke `gocyclo` from orbit and everything in the world will be much better for it.

* Review comments

roomserver/api/api_trace.go

@@ -354,6 +354,16 @@ func (t *RoomserverInternalAPITrace) QueryAuthChain(
 	return err
 }
 
+func (t *RoomserverInternalAPITrace) QueryRestrictedJoinAllowed(
+	ctx context.Context,
+	request *QueryRestrictedJoinAllowedRequest,
+	response *QueryRestrictedJoinAllowedResponse,
+) error {
+	err := t.Impl.QueryRestrictedJoinAllowed(ctx, request, response)
+	util.GetLogger(ctx).WithError(err).Infof("QueryRestrictedJoinAllowed req=%+v res=%+v", js(request), js(response))
+	return err
+}
+
 func js(thing interface{}) string {
 	b, err := json.Marshal(thing)
 	if err != nil {