Show problems to object owners even without mod permission

[Floppy]

Contributors can't see Problems any more after a blunt resolution to #4353, but ideally they should be able to see the problems with their own models.

The logic here is complex and expensive; I think I might just restrict problem visibility to moderators and above for now, and hide it for contributors, even on their own models (which isn't ideal but manageable in the short term).

Originally posted by @Floppy in #4353