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/plugin-drawio.js

@@ -135,9 +135,9 @@ export function getPlugin(providedOptions) {
             if (!drawings.length) return;
 
             editor.undoManager.transact(function () {
-                drawings.each((index, elem) => {
-                    elem.setAttribute('contenteditable', 'false');
-                });
+                for (const drawing of drawings) {
+                    drawing.setAttribute('contenteditable', 'false');
+                }
             });
         });