[inferno-ml] Simplify model permissions, add more metadata (#132)

Includes several changes to the `Model` and related types:

- Model permissions are now expressed as a single group ID (which
simplifies things considerably)
- Models now contain `visibility` and `updated` fields, similar to
Inferno VC scripts
- Some smaller changes to `ModelVersion` (adding a high-level
description, etc...)
- Removal of `users` table, which isn't necessary

Since we don't need to support GHC 8 any longer, I've switched to using
`-XOverloadedRecordDot` in some places. I'm going to remove GHC 8
support entirely when I finish the compiler upgrade (blocked on NVIDIA
pain at the moment).