Move spec permalinks into their own class

This allows for Riot permalinks to be introduced without if-else ladders everywhere.

src/utils/permalinks/PermalinkConstructor.js

@@ -0,0 +1,37 @@
+/*
+Copyright 2019 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.
+*/
+
+/**
+ * Interface for classes that actually produce permalinks (strings).
+ * TODO: Convert this to a real TypeScript interface
+ */
+export default class PermalinkConstructor {
+    forEvent(roomId: string, eventId: string, serverCandidates: string[]): string {
+        throw new Error("Not implemented");
+    }
+
+    forRoom(roomIdOrAlias: string, serverCandidates: string[]): string {
+        throw new Error("Not implemented");
+    }
+
+    forGroup(groupId: string): string {
+        throw new Error("Not implemented");
+    }
+
+    forUser(userId: string): string {
+        throw new Error("Not implemented");
+    }
+}