Editing multiple contributions pages at the same time results in overwrite

Tested on sandbox.

• Open contribution page 1 for editing in browser tab A
• Open contribution page 2 for editing in browser tab B
• Click "save" in browser tab A

The contents of contribution page 1 overwrite those of contribution page 2, resulting in a duplicate. In other words, you can't have multiple contribution page forms open for editing at the same time.

(Called it a "major" priority because it's common to copy the settings from one page to another, so admins often open both.)