Make the hangup button do things for conference calls

Behaviour constraints:
* If you're not in the conference, use a grey button that does nothing.
* If you're in the conference, show a button:
  * If you're able to modify widgets in the room, annotate it in the context of ending the call for everyone and remove the widget. Use a confirmation dialog.
  * If you're not able to modify widgets in the room, hang up.

For this we know that persistent Jitsi widgets will mean that the user is in the call, so we use that to determine if they are actually participating.

src/WidgetMessaging.js

@@ -107,6 +107,17 @@ export default class WidgetMessaging {
         });
     }
 
+    /**
+     * Tells the widget to hang up on its call.
+     * @returns {Promise<*>} Resolves when teh widget has acknowledged the message.
+     */
+    hangup() {
+        return this.messageToWidget({
+            api: OUTBOUND_API_NAME,
+            action: KnownWidgetActions.Hangup,
+        });
+    }
+
     /**
      * Request a screenshot from a widget
      * @return {Promise} To be resolved with screenshot data when it has been generated