This PR introduces a `UserService` trait to Collab.
This is a step towards moving Collab away from reading user information
directly from the database.
We currently have two implementations for the trait:
- The `DatabaseUserService`, which leverages the existing query methods
to talk to the database
- The `FakeUserService`, which will be used in tests
Once we're ready, we'll be able to replace the `DatabaseUserService`
with a `CloudUserService` to fetch the users from Cloud.
Release Notes:
- N/A
Marshall Bowers
01702ee309