Made a bunch of tinymce 6 upgrade fixes

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.