Same holder as owner makes item unusable

[IsThisTheCrustyCrab-was-taken]

Bug: Same holder as owner makes item unusable

Problem Description

When creating an item and choosing the same user for holder and owner the item now becomes locked to that holder and doesn't seem to be able to be unlended.

1. What happens: A user creates an item and sets i.e. themself as owner and holder
2. What is the expected behaviour: Either the holder cannot be set to owner or the rental can be ended.
3. Why is this a problem: Users can accidentally create unlendable items.

Steps For Reproduction

1. create new item
2. set self as owner and holder
3. other users now cannot request to borrow the item and you cannot stop your own rental