Reject make_join for empty rooms (#1439)

* Sanity-check room version on RS event input

* Update gomatrixserverlib

* Reject make_join when no room members are left

* Revert some changes from wrong branch

* Distinguish between room not existing and room being abandoned on this server

* nolint

roomserver/api/api.go

@@ -89,6 +89,13 @@ type RoomserverInternalAPI interface {
 		response *QueryMembershipsForRoomResponse,
 	) error
 
+	// Query if we think we're still in a room.
+	QueryServerJoinedToRoom(
+		ctx context.Context,
+		request *QueryServerJoinedToRoomRequest,
+		response *QueryServerJoinedToRoomResponse,
+	) error
+
 	// Query whether a server is allowed to see an event
 	QueryServerAllowedToSeeEvent(
 		ctx context.Context,