Tweak roles: Add "Director" position, remove "Admin"

[peterkos]

Old structure:
User -> event_tracking -> admin_limited_access -> Admin

New structure:
User -> Volunteer -> Organizer -> Director

Directors can modify the permissions of organizers, while organizers cannot modify other organizers' permissions.

This will allow for more role customization down the road.

[peterkos]

Migrations and API will have to be given careful consideration. See #222 for a similar large, breaking change for how that was implemented in its linked PRs.

[peterkos]

Note that this will be artifically blocked until #222 is done. One large refactor at a time!