Add Undo option for restoring unsaved changes in case of unwanted modifications when editing a page in realtime

Steps to reproduce

1. Go to a page with some content (e.g. Sandbox)
2. Edit in realtime the page (in-place or in WYSIWYG mode)
3. Delete all content on the page (or make any other modification)
4. Click on another link (e.g. Dashboard or Home) to navigate away without saving (without clicking "Done")
5. Go back to the previously modified page (e.g. Sandbox)
6. Click 'Edit' button to edit the page again (in-place or in WYSIWYG mode)
7. Observe the restored content and the available options

Expected results

Along with the "Unsaved changes restored" message, there is an "Undo" button for user to be able to undo restoration of the previously deleted/ modified content (which may have been done by mistake).

Actual results

The page is empty and even if the page is edited multiple times in-place or in WYSIWYG mode, the content is still missing in edit mode.

What I've noticed is that after editing the page multiple times one after another (app. 16 times), the message "Unsaved changes restored" is not displayed anymore (this may not be a much used case, but it's worth mentioning).