Fixes #16853: Provide a way to disable relationships feature.

Add configuration to enable/disable relationship feature as well as define the threshold for users to update relationships independent on the generic update threshold.

I started off with ON/OFF, then thought about implementing a threshold to control that, but that would complicate the filtering story for all projects, etc. Hence, I opted for ON/OFF and either adding a new threshold for edit or I'm OK with just going back to the generic update threshold.