Add feature flag for polls and placeholder dialog

We're anticipating development of polls soon, so for efficiency reasons we're getting the labs flag in early. If something goes wrong with the plan, this whole change will be reverted.

src/settings/Settings.tsx

@@ -246,6 +246,12 @@ export const SETTINGS: {[setting: string]: ISetting} = {
         default: false,
         controller: new PseudonymousAnalyticsController(),
     },
+    "feature_polls": {
+        isFeature: true,
+        supportedLevels: LEVELS_FEATURE,
+        displayName: _td("Polls (under active development)"),
+        default: false,
+    },
     "doNotDisturb": {
         supportedLevels: [SettingLevel.DEVICE],
         default: false,