Updated activity feed to use full width

no issue

- Activity feed page now has `gh-main-fullwidth` class
- Page `<title>` is now 'Activity - Ghost' instead of just 'Ghost'

ghost/admin/app/routes/members-activity.js

@@ -1,4 +1,10 @@
 import AdminRoute from 'ghost-admin/routes/admin';
 
 export default class MembersActivityRoute extends AdminRoute {
+    buildRouteInfoMetadata() {
+        return {
+            titleToken: 'Activity',
+            mainClasses: ['gh-main-fullwidth']
+        };
+    }
 }