Add `&` support for epics autocompletion

app/assets/javascripts/gfm_auto_complete.js

 import $ from 'jquery';
 import _ from 'underscore';
+
+// EE-specific
+import setupAutoCompleteEpics from 'ee/gfm_auto_complete_ee';
+
 import glRegexp from './lib/utils/regexp';
 import AjaxCache from './lib/utils/ajax_cache';
 
@@ -51,6 +55,9 @@ class GfmAutoComplete {
     if (this.enableMap.mergeRequests) this.setupMergeRequests($input);
     if (this.enableMap.labels) this.setupLabels($input);
 
+    // EE-specific
+    if (this.enableMap.epics) setupAutoCompleteEpics($input, this.getDefaultCallbacks());
+
     // We don't instantiate the quick actions autocomplete for note and issue/MR edit forms
     $input.filter('[data-supports-quick-actions="true"]').atwho({
       at: '/',

ee/app/assets/javascripts/gfm_auto_complete_ee.js

+import $ from 'jquery';
+import GfmAutoComplete from '~/gfm_auto_complete';
+
+const setupAutoCompleteEpics = ($input, defaultCallbacks) => {
+  $input.atwho({
+    at: '&',
+    alias: 'epics',
+    searchKey: 'search',
+    displayTpl(value) {
+      let tmpl = GfmAutoComplete.Loading.template;
+      if (value.title != null) {
+        tmpl = GfmAutoComplete.Issues.template;
+      }
+      return tmpl;
+    },
+    data: GfmAutoComplete.defaultLoadingData,
+    // eslint-disable-next-line no-template-curly-in-string
+    insertTpl: '${atwho-at}${id}',
+    callbacks: {
+      ...defaultCallbacks,
+      beforeSave(merges) {
+        return $.map(merges, (m) => {
+          if (m.title == null) {
+            return m;
+          }
+          return {
+            id: m.iid,
+            title: m.title.replace(/<(?:.|\n)*?>/gm, ''),
+            search: `${m.iid} ${m.title}`,
+          };
+        });
+      },
+    },
+  });
+};
+
+export default setupAutoCompleteEpics;