A role given to someone on a record: a row of sekisho_grants.
actor_type is the grantee's table ("users", "teams"), or "link"
for anyone with the link, whose token is set only in what grant/4
returns: the table keeps its hash.
The ids are of the type of your app's records (bigint or uuid, see
Sekisho.Migrations), so the grants join your tables on their indexes.