Only renders notes tab when viewing discussions

Instead of always rendering the overview tab, this changes it to
only render the discussions whenever the user is viewing the overview
tab. Doing this helps save memory, especially on much larger
merge requests.

app/assets/javascripts/notes/components/notes_app.vue

@@ -66,6 +66,7 @@ export default {
   data() {
     return {
       currentFilter: null,
+      renderSkeleton: !this.shouldShow,
     };
   },
   computed: {
@@ -93,7 +94,7 @@ export default {
       return this.noteableData.noteableType;
     },
     allDiscussions() {
-      if (this.isLoading) {
+      if (this.renderSkeleton || this.isLoading) {
         const prerenderedNotesCount = parseInt(this.notesData.prerenderedNotesCount, 10) || 0;
 
         return new Array(prerenderedNotesCount).fill({
@@ -122,6 +123,10 @@ export default {
       if (!this.isNotesFetched) {
         this.fetchNotes();
       }
+
+      setTimeout(() => {
+        this.renderSkeleton = !this.shouldShow;
+      });
     },
     discussionTabCounterText(val) {
       if (this.discussionsCount) {