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/config.js

@@ -121,7 +121,7 @@ function setupBrFilter(editor) {
     editor.serializer.addNodeFilter('br', function(nodes) {
         for (const node of nodes) {
             if (node.parent && node.parent.name === 'code') {
-                const newline = new tinymce.html.Node.create('#text');
+                const newline = tinymce.html.Node.create('#text');
                 newline.value = '\n';
                 node.replace(newline);
             }