Add autocompletion in textarea for user mentions

assets/js/src/Dropdown.js

@@ -40,6 +40,21 @@ Dropdown.prototype.listen = function() {
             $(this).find('a:visible')[0].click(); // Calling native click() not the jQuery one
         }
     });
+
+    // User mention autocomplete
+    $('textarea[data-mention-search-url]').textcomplete([{
+        match: /(^|\s)@(\w*)$/,
+        search: function (term, callback) {
+            var url = $('textarea[data-mention-search-url]').data('mention-search-url');
+            $.getJSON(url, { q: term })
+                .done(function (resp) { callback(resp); })
+                .fail(function ()     { callback([]);   });
+        },
+        replace: function (value) {
+            return '$1@' + value + ' ';
+        },
+        cache: true
+    }], {className: "textarea-dropdown"});
 };
 
 Dropdown.prototype.close = function() {