Steps to replicate the issue (include links if applicable):

• Create an event page and enable registration
• Move the event page; you can leave the title as-is, but choose a namespace other than "Event"

What happens?:
The move goes through. If you go to the new page, the registration header isn't shown; however, you can verify in the database that the registration has indeed been associated with the non-Event page.

Further, if you move the page back to the Event namespace (leaving a redirect behind), the move will go through but the registration will remain associated with the non-Event page (i.e., it's not moved automatically).

What should have happened instead?:
Good question. Since we chose to not allow event registration to be enabled on page outside of the Event namespace, I think we should disallow moving a page to outside the namespace if it has a registration associated.

If we choose to do T318179 (allow enabling registration outside the Event namespace), then I think we should just remove this limitation, and make sure that the registration header shows as normal regardless of the namespace.

Software version (skip for WMF-hosted wikis like Wikipedia): master

Other information (browser name/version, screenshots, etc.): ∅