Check and update module for SS4 compatibility where necessary

[rupachup]

Acceptance Criteria

• [x] Works on SS4
• [x] Beta tag released

Sub tasks

• [x] #159
• [x] Saving a page with new widgets redirects to 404 couldn't reproduce

Pull requests

• [x] #161

[robbieaverill]

https://github.com/silverstripe/silverstripe-widgets/releases/tag/2.0.0-beta1