Deal with missing registration profiles (forms)

[bjendres]

The system allows you to provide custom event registration profiles trough other extensions, but it is currently not clear what should happen if these profiles are removed (e.g. by disabling the extension) while the event still exists.

[jensschuppe]

This might become more obvious with https://github.com/systopia/remoteeventformeditor being used, which allows to delete profiles via the UI. Ping @dontub.

[jensschuppe]

Moving to 1.3 as 1.2 is already in beta.

[dontub]

There should be a way to detect if and how many times a profile is used to prevent deletion.

[jensschuppe]

What about profiles provided by extensions that get uninstalled? Do we want to add a system check for that (checking events active for registration with profiles that can't be found)?