Use UUID instead of a sequential numeric ID to identify pretenders

As we move towards using shared hosting, I though one simple improvement to help isolate pretenders is to use random uuid values instead of sequential IDs to identify them.

This way, without needing to layer some authentication and pretender isolation per user on top of the existing solution, we can have some level of guarantee that one can't easily meddle with someone else's mock.

We will need to remove the ability to list pretenders in the UI for now, for any shared host solution.