Supervisor can view all volunteers

[Zrrrpy]

Part of epic #105

What type of user is this for? [admin/volunteer/supervisor/all]
Supervisor

Where does/should this occur?
In the Supervisor dashboard (which doesn't exist yet, but is ready to!) – it will look almost identical to the Admin dashboard.

Description
Supervisors should be able to view all volunteers, but their default view should be filtered down to only volunteers assigned to them. Otherwise, supervisors should be able to filter volunteers just like admins can (by supervisor, volunteer status, transition aged youth).

Additional details about supervisor permissions can be found in issue #30.

Screenshots
It should look like this, except supervisors will NOT see a button that says "New Volunteer" at the top.

Relates to issues
#17, #20, #30