add loading spinner to sidebar assignees

app/assets/javascripts/sidebar/components/assignees/sidebar_assignees.js

@@ -79,6 +79,7 @@ export default {
         :editable="store.editable"
         @assign-self="assignSelf"
       />
+      <i class="fa fa-spinner fa-spin"/>
     </div>
   `,
 };

app/views/shared/issuable/_sidebar_assignees.html.haml

 - if issuable.is_a?(Issue)
   #js-vue-sidebar-assignees{ data: { field: "#{issuable.to_ability_name}[assignee_ids]" } }
+    .title.hide-collapsed
+      Assignee
+      = icon('spinner spin', 'aria-hidden': 'true')
 - else
   .sidebar-collapsed-icon.sidebar-collapsed-user{ data: { toggle: "tooltip", placement: "left", container: "body" }, title: (issuable.assignee.name if issuable.assignee) }
     - if issuable.assignee