FEATURE: site setting enable_mentions to turn on/off mentions

app/assets/javascripts/discourse/components/composer-editor.js.es6

@@ -77,16 +77,18 @@ export default Ember.Component.extend({
     const $input = this.$('.d-editor-input');
     const $preview = this.$('.d-editor-preview-wrapper');
 
-    $input.autocomplete({
-      template: findRawTemplate('user-selector-autocomplete'),
-      dataSource: term => userSearch({
-        term,
-        topicId,
-        includeMentionableGroups: true
-      }),
-      key: "@",
-      transformComplete: v => v.username || v.name
-    });
+    if (this.siteSettings.enable_mentions) {
+      $input.autocomplete({
+        template: findRawTemplate('user-selector-autocomplete'),
+        dataSource: term => userSearch({
+          term,
+          topicId,
+          includeMentionableGroups: true
+        }),
+        key: "@",
+        transformComplete: v => v.username || v.name
+      });
+    }
 
     if (this._enableAdvancedEditorPreviewSync()) {
       this._initInputPreviewSync($input, $preview);

app/assets/javascripts/discourse/lib/search.js.es6

@@ -154,13 +154,15 @@ export function applySearchAutocomplete($input, siteSettings, appEvents, options
     afterComplete
   }, options));
 
-  $input.autocomplete(_.merge({
-    template: findRawTemplate('user-selector-autocomplete'),
-    key: "@",
-    width: '100%',
-    treatAsTextarea: true,
-    transformComplete: v => v.username || v.name,
-    dataSource: term => userSearch({ term, includeGroups: true }),
-    afterComplete
-  }, options));
+  if (Discourse.SiteSettings.enable_mentions) {
+    $input.autocomplete(_.merge({
+      template: findRawTemplate('user-selector-autocomplete'),
+      key: "@",
+      width: '100%',
+      treatAsTextarea: true,
+      transformComplete: v => v.username || v.name,
+      dataSource: term => userSearch({ term, includeGroups: true }),
+      afterComplete
+    }, options));
+  }
 };

app/assets/javascripts/pretty-text/engines/discourse-markdown/mentions.js.es6

@@ -38,6 +38,10 @@ function addMention(buffer, matches, state) {
 }
 
 export function setup(helper) {
+  helper.registerOptions((opts, siteSettings) => {
+    opts.features.mentions = !!siteSettings.enable_mentions;
+  });
+
   helper.registerPlugin(md => {
 
     const rule = {