Stop multiple discussion requests

With multiple discussion requests we loose the reactivity of the
previously stored discussions state.
This means that certain features, toggling line discussions for example,
are no longer reactive.

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

@@ -50,6 +50,7 @@ export default {
   },
   data() {
     return {
+      isFetching: false,
       currentFilter: null,
     };
   },
@@ -141,6 +142,10 @@ export default {
       return discussion.individual_note ? { note: discussion.notes[0] } : { discussion };
     },
     fetchNotes() {
+      if (this.isFetching) return null;
+
+      this.isFetching = true;
+
       return this.fetchDiscussions({ path: this.getNotesDataByProp('discussionsPath') })
         .then(() => {
           this.initPolling();
@@ -149,6 +154,7 @@ export default {
           this.setLoadingState(false);
           this.setNotesFetchedState(true);
           eventHub.$emit('fetchedNotesData');
+          this.isFetching = false;
         })
         .then(() => this.$nextTick())
         .then(() => this.checkLocationHash())