Volunteers: allow shift management access for people outside concom

Currently, volunteer shift management is only accessible to concom members. Event organizers want to delegate some shift management tasks (e.g. editing shift lists) to people outside the core concom.

## Requested behaviour

Allow granting shift management access to people who are not full concom members, scoped to specific departments or shift lists.

## Background

Requested in customer feedback. Part of the broader CBAC access control work.