Removing the shortcode from the tickets page, or changing its slug or publicity breaks the ability to refund tickets, etc.

This PR aims to prevent removal of camptix shortcode, change in page publicity or slug when tickets have been sold.

Fixes #402

Props @iandunn

TODO

• Add notification on edit view to tell that tickets have been sold so these actions cannot be taken
• Test Gutenberg
• Move the function to a better place

Screenshots

Warning when changes are prevented, block editor.

Warning when changes are prevented, classic editor.

Error when changes are tried, block editor.

Error when changes are tried, classic editor.

How to test the changes in this Pull Request:

1. Open WordCamp site dashboard, ensure that no tickets have been sold and tickets page is draft
2. Update the tickets page, and change slug for example -> updating should work
3. Publish the page, remove camptix shortcode this time -> updating should work
4. Add the shortcode back and buy one ticket
5. Open the tickets page in classic editor -> you should see warning
6. Open the tickets page in block editor -> you should see warning
7. Update the page on block editor, change slug -> you should get error
8. Update the page on block editor, remove camptix shortcode -> you should get error
9. Update the page on classic editor, try to change staus back to draft -> you should get error