update shortcuts scripts to use ES module syntax

app/assets/javascripts/shortcuts_blob.js

 /* global Mousetrap */
 /* global Shortcuts */
-require('./shortcuts');
+import './shortcuts';
 const defaults = {
   skipResetBindings: false,

app/assets/javascripts/shortcuts_find_file.js

@@ -2,7 +2,7 @@
 /* global Mousetrap */
 /* global ShortcutsNavigation */
-require('./shortcuts_navigation');
+import './shortcuts_navigation';
 (function() {
   var extend = function(child, parent) { for (var key in parent) { if (hasProp.call(parent, key)) child[key] = parent[key]; } function ctor() { this.constructor = child; } ctor.prototype = parent.prototype; child.prototype = new ctor(); child.__super__ = parent.prototype; return child; },

app/assets/javascripts/shortcuts_issuable.js

@@ -3,8 +3,8 @@
 /* global ShortcutsNavigation */
 /* global sidebar */
-require('mousetrap');
-require('./shortcuts_navigation');
+import 'mousetrap';
+import './shortcuts_navigation';
 (function() {
   var extend = function(child, parent) { for (var key in parent) { if (hasProp.call(parent, key)) child[key] = parent[key]; } function ctor() { this.constructor = child; } ctor.prototype = parent.prototype; child.prototype = new ctor(); child.__super__ = parent.prototype; return child; },

app/assets/javascripts/shortcuts_navigation.js

 /* eslint-disable func-names, space-before-function-paren, max-len, no-var, one-var, no-restricted-syntax, vars-on-top, no-use-before-define, no-param-reassign, new-cap, no-underscore-dangle, wrap-iife, prefer-arrow-callback, consistent-return, no-return-assign */
 /* global Mousetrap */
 /* global Shortcuts */
-import findAndFollowLink from './shortcuts_dashboard_navigation';
-require('./shortcuts');
+import findAndFollowLink from './shortcuts_dashboard_navigation';
+import './shortcuts';
 (function() {
   var extend = function(child, parent) { for (var key in parent) { if (hasProp.call(parent, key)) child[key] = parent[key]; } function ctor() { this.constructor = child; } ctor.prototype = parent.prototype; child.prototype = new ctor(); child.__super__ = parent.prototype; return child; },

app/assets/javascripts/shortcuts_network.js

@@ -2,7 +2,7 @@
 /* global Mousetrap */
 /* global ShortcutsNavigation */
-require('./shortcuts_navigation');
+import './shortcuts_navigation';
 (function() {
   var extend = function(child, parent) { for (var key in parent) { if (hasProp.call(parent, key)) child[key] = parent[key]; } function ctor() { this.constructor = child; } ctor.prototype = parent.prototype; child.prototype = new ctor(); child.__super__ = parent.prototype; return child; },