Fix #835: Hide pod statistics on about page if those are disabled

changes/changelog.d/835.bugfix

+Hide pod statistics on about page if those are disabled (#835)

front/src/components/instance/Stats.vue

 <template>
   <div>
-    <div v-if="stats" class="ui stackable two column grid">
+    <div v-if="stats && stats.trackFavorites !== undefined" class="ui stackable two column grid">
       <div class="column">
         <h3 class="ui left aligned header">
           <translate translate-context="Content/About/Title/Noun">User activity</translate>