Prevent creating multiple pages, codes, and blocks when editing a Sitewide Sale

[dparker1005]

All Submissions:

• [x] Have you followed the Contributing guideline?
• [x] Does your code follow the WordPress' coding standards?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Disable the "Create a new X" button" after it has been clicked on the "Edit Sitewide Sale" page to avoid situations where administrators create duplicate pages, codes, or blocks.

Other information:

• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [x] Have you successfully run tests with your changes locally?

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.