Implement /createRoom power_level_content_override (#1761)

* Implement /createRoom power_level_content_override

* delint

* Skip having to re-marshal PowerLevelContentOverride by making use of `json.RawMessage`

clientapi/routing/createroom.go

@@ -48,6 +48,7 @@ type createRoomRequest struct {
 	RoomAliasName             string                        `json:"room_alias_name"`
 	GuestCanJoin              bool                          `json:"guest_can_join"`
 	RoomVersion               gomatrixserverlib.RoomVersion `json:"room_version"`
+	PowerLevelContentOverride json.RawMessage               `json:"power_level_content_override"`
 }
 
 const (
@@ -257,6 +258,18 @@ func createRoom(
 
 	var builtEvents []*gomatrixserverlib.HeaderedEvent
 
+	powerLevelContent := eventutil.InitialPowerLevelsContent(userID)
+	if r.PowerLevelContentOverride != nil {
+		// Merge powerLevelContentOverride fields by unmarshalling it atop the defaults
+		err = json.Unmarshal(r.PowerLevelContentOverride, &powerLevelContent)
+		if err != nil {
+			return util.JSONResponse{
+				Code: http.StatusBadRequest,
+				JSON: jsonerror.BadJSON("malformed power_level_content_override"),
+			}
+		}
+	}
+
 	// send events into the room in order of:
 	//  1- m.room.create
 	//  2- room creator join member
@@ -278,7 +291,7 @@ func createRoom(
 	eventsToMake := []fledglingEvent{
 		{"m.room.create", "", r.CreationContent},
 		{"m.room.member", userID, membershipContent},
-		{"m.room.power_levels", "", eventutil.InitialPowerLevelsContent(userID)},
+		{"m.room.power_levels", "", powerLevelContent},
 		{"m.room.join_rules", "", gomatrixserverlib.JoinRuleContent{JoinRule: joinRules}},
 		{"m.room.history_visibility", "", eventutil.HistoryVisibilityContent{HistoryVisibility: historyVisibility}},
 	}