Feature summary (what you would like to be able to do and where):
Abusefilter should request a confirmation (interstitial) from the user when the user attempts to make a private filter into a public one. (a la the confirmation that displays for a purge action)

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
See T391509 where I apparently accidentally made a filter that was private public instead.

Benefits (why should this be implemented?):
Avoids accidental disclosure of private filter implementation.