Make it possible in-code to hide rooms from the room list

Fixes https://github.com/vector-im/element-web/issues/15745

This was surprisingly easy given the number of errors I remember last time, but here it is. This also includes an over-engineered VisibilityProvider with the intention that it'll get used in the future for things like Spaces and other X as Rooms stuff.

src/customisations/RoomList.ts

@@ -0,0 +1,45 @@
+/*
+ * Copyright 2020 The Matrix.org Foundation C.I.C.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *         http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { Room } from "matrix-js-sdk/src/models/room";
+
+// Populate this file with the details of your customisations when copying it.
+
+/**
+ * Determines if a room is visible in the room list or not. By default,
+ * all rooms are visible. Where special handling is performed by Element,
+ * those rooms will not be able to override their visibility in the room
+ * list - Element will make the decision without calling this function.
+ *
+ * This function should be as fast as possible to avoid slowing down the
+ * client.
+ * @param {Room} room The room to check the visibility of.
+ * @returns {boolean} True if the room should be visible, false otherwise.
+ */
+function isRoomVisible(room: Room): boolean {
+    return true;
+}
+
+// This interface summarises all available customisation points and also marks
+// them all as optional. This allows customisers to only define and export the
+// customisations they need while still maintaining type safety.
+export interface IRoomListCustomisations {
+    isRoomVisible?: typeof isRoomVisible;
+}
+
+// A real customisation module will define and export one or more of the
+// customisation points that make up the interface above.
+export const RoomListCustomisations: IRoomListCustomisations = {};