UI Feature Flag: Disable advanced options and tidy up some copy

src/settings/Settings.ts

@@ -622,4 +622,8 @@ export const SETTINGS: {[setting: string]: ISetting} = {
         supportedLevels: LEVELS_UI_FEATURE,
         default: true,
     },
+    [UIFeature.AdvancedSettings]: {
+        supportedLevels: LEVELS_UI_FEATURE,
+        default: true,
+    },
 };

src/settings/UIFeature.ts

@@ -18,4 +18,5 @@ limitations under the License.
 export enum UIFeature {
     URLPreviews = "UIFeature.urlPreviews",
     Widgets = "UIFeature.widgets",
+    AdvancedSettings = "UIFeature.advancedSettings",
 }