Labs: Add quick/cheap "do not disturb" flag

This just disables audio notifications and the popup, which is the easiest way to do "do not disturb" for a device. This needs spec changes to be done properly, as it's a shame that mobile devices for the user will still go off.

Disabling all of push doesn't sound ideal as it would potentially mean missing highlights for when leaving DND mode.

src/settings/Settings.ts

@@ -128,6 +128,12 @@ export const SETTINGS: {[setting: string]: ISetting} = {
         default: false,
         controller: new ReloadOnChangeController(),
     },
+    "feature_dnd": {
+        isFeature: true,
+        displayName: _td("Show options to enable 'Do not disturb' mode"),
+        supportedLevels: LEVELS_FEATURE,
+        default: false,
+    },
     "feature_voice_messages": {
         isFeature: true,
         displayName: _td("Send and receive voice messages (in development)"),
@@ -226,6 +232,10 @@ export const SETTINGS: {[setting: string]: ISetting} = {
         supportedLevels: LEVELS_DEVICE_ONLY_SETTINGS,
         default: false,
     },
+    "doNotDisturb": {
+        supportedLevels: [SettingLevel.DEVICE],
+        default: false,
+    },
     "mjolnirRooms": {
         supportedLevels: [SettingLevel.ACCOUNT],
         default: [],