Made a bunch of tinymce 6 upgrade fixes

- Added workaround for new 'srcdoc' usage that's breaking content in
  Firefox, added new 'custom-changes.md' file to document for future.
- Updated old usages of 'new' when creating nodes.
- Tested and changed logic, where required, where 'editor.dom.select'
  has been used to replace the old '$' usages.
- Fixed bad boolean value being passed to 'setActive' in task list
  logic.

resources/js/wysiwyg/drop-paste-handling.js

@@ -132,9 +132,9 @@ function drop(editor, options, event) {
             const range = editor.selection.getRng();
             const selectedNodeRoot = selectedNode.closest('body > *');
             if (range.startOffset > (range.startContainer.length / 2)) {
-                editor.dom.select(selectedNodeRoot).after(draggedContentEditable);
+                selectedNodeRoot.after(draggedContentEditable);
             } else {
-                editor.dom.select(selectedNodeRoot).before(draggedContentEditable);
+                selectedNodeRoot.before(draggedContentEditable);
             }
         });
     }