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.

public/libs/tinymce/custom-changes.md

@@ -0,0 +1,17 @@
+
+
+### Srcdoc usage
+
+By default, as of tiny 6, the editor would use srcdoc which prevents cookies being sent with images in Firefox as 
+it's considered cross origin. This removes that usage to work around this case:
+
+[Relevant TinyMCE issue](https://github.com/tinymce/tinymce/issues/7746).
+
+Source code change applied:
+
+```javascript
+// Find:
+t.srcdoc=e.iframeHTML
+// Replace:
+t.contentDocument.open();t.contentDocument.write(e.iframeHTML);t.contentDocument.close();
+```