🎉 Support ActiveAdmin 2.x and code style improvements

- Add support ActiveAdmin for 2.x
- Add rubocop config
- Add frozen_string_literal to ruby files
- Apply changes to respect rubocop alerts

app/assets/javascripts/activeadmin/quill_editor_input.js

@@ -1,4 +1,4 @@
-window.onload = function () {
+window.onload = function() {
   initQuillEditors();
 }
 
@@ -7,7 +7,7 @@ $(document).on('has_many_add:after', function() {
 });
 
 var initQuillEditors = function() {
-  var editors = document.querySelectorAll( '.quill-editor' );
+  var editors = document.querySelectorAll('.quill-editor');
   var default_options = {
     modules: {
       toolbar: [
@@ -17,32 +17,28 @@ var initQuillEditors = function() {
         [{ 'align': [] }, { list: 'ordered' }, { list: 'bullet' }],
         [{ 'color': [] }, { 'background': [] }],
         ['clean'],
-        // [{ 'size': ['small', false, 'large', 'huge'] }],
-        // [{ 'header': [1, 2, 3, 4, 5, 6, false] }],
       ]
     },
     placeholder: '',
     theme: 'snow'
   };
 
-  for( var i = 0; i < editors.length; i++ ) {
-    var content = editors[i].querySelector( '.quill-editor-content' );
+  for(var i = 0; i < editors.length; i++) {
+    var content = editors[i].querySelector('.quill-editor-content');
     var isActive = editors[i].classList.contains('quill-editor--active');
-    if( content && !isActive) {
-      var options = editors[i].getAttribute( 'data-options' ) ? JSON.parse( editors[i].getAttribute( 'data-options' ) ) : default_options;
-      editors[i]['_quill-editor'] = new Quill( content, options );
+    if(content && !isActive) {
+      var options = editors[i].getAttribute('data-options') ? JSON.parse(editors[i].getAttribute('data-options')) : default_options;
+      editors[i]['_quill-editor'] = new Quill(content, options);
       editors[i].classList += ' quill-editor--active';
     }
   }
 
-  var formtastic = document.querySelector( 'form.formtastic' );
-  if( formtastic ) {
+  var formtastic = document.querySelector('form.formtastic');
+  if(formtastic) {
     formtastic.onsubmit = function() {
-      for( var i = 0; i < editors.length; i++ ) {
-        var input = editors[i].querySelector( 'input[type="hidden"]' );
+      for(var i = 0; i < editors.length; i++) {
+        var input = editors[i].querySelector('input[type="hidden"]');
         input.value = editors[i]['_quill-editor'].root.innerHTML;
-        // input.value = editors[i]['_quill-editor'].getContents();
-        // input.value = editors[i].querySelector( '.quill-editor-content' ).innerHTML;
       }
     };
   }