Introduce a concept of UI features, using it for URL previews at first

Fixes https://github.com/vector-im/element-web/issues/15176

This is effectively the base for all of https://github.com/vector-im/element-web/issues/15185

src/settings/UIFeature.ts

@@ -0,0 +1,20 @@
+/*
+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.
+*/
+
+// see settings.md for documentation on conventions
+export enum UIFeature {
+    URLPreviews = "UIFeature.URLPreviews",
+}