Fix seen by and "unread messages" marker

It was broken by the "filter out status messages" functionality.

src/components/messages/SeenBy.vue

@@ -1,6 +1,6 @@
 <template>
   <div class="seen-by-container">
-    <v-tooltip top open-delay="500">
+    <v-tooltip top open-delay="500" v-if="seenBy.length > 0">
       <template v-slot:activator="{ on, attrs }">
         <div v-bind="attrs" v-on="on" class="clickable">
           <div class="more" v-if="seenBy.length > 0">{{ moreItems }}</div>