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/controllers/UIFeatureController.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 SettingController from "./SettingController";
+import { SettingLevel } from "../SettingLevel";
+import SettingsStore from "../SettingsStore";
+
+/**
+ * Enforces that a boolean setting cannot be enabled if the corresponding
+ * UI feature is disabled. If the UI feature is enabled, the setting value
+ * is unchanged.
+ *
+ * Settings using this controller are assumed to return `false` when disabled.
+ */
+export default class UIFeatureController extends SettingController {
+    public constructor(private uiFeatureName: string) {
+        super();
+    }
+
+    public getValueOverride(
+        level: SettingLevel,
+        roomId: string,
+        calculatedValue: any,
+        calculatedAtLevel: SettingLevel,
+    ): any {
+        if (!SettingsStore.getValue(this.uiFeatureName)) {
+            // per the docs: we force a disabled state when the feature isn't active
+            return false;
+        }
+        return null; // no override
+    }
+}