Debounce $scope updates when typing in chat. (#8485)

Fixes #6462, by saving a bunch of time per frame. See the issue for evidence of
the win.

website/client-old/js/directives/submit-form-on-enter.directive.js

@@ -0,0 +1,25 @@
+'use strict';
+
+(function(){
+
+  angular
+    .module('habitrpg')
+    .directive('submitOnMetaEnter', submitOnMetaEnter);
+
+  function submitOnMetaEnter() {
+    return {
+      restrict: 'A',
+      link: function($scope, element, attrs) {
+        element.on('keydown', function(event) {
+          if (event.key === 'Enter' && (event.metaKey || event.ctrlKey)) {
+            // Note that we use the normal browser way to invoke the submit
+            // event, because jquery's triggerHandler executes events in a
+            // strange order!
+            var event = new Event('submit');
+            element[0].form.dispatchEvent(event);
+          }
+        });
+      }
+    }
+  }
+}());