authz: simplify `PermsStore` logic with intarray

Created by: unknwon

Uses operators and functions provided by Postgres' intarray extension to greatly simplify the logic of some PermsStore methods.

Regarding in-place changes in tests, I got confused in debugging SQLs with these IDs, so I choose to use mutually exclusive sets of IDs for users and repos.

Please review by commit in split view

Stack on https://github.com/sourcegraph/sourcegraph/pull/11457 and fixes #11768