Volunteer teams

[jellybob]

This ballooned somewhat, so apologies for the messy commit history. I did what I could with rebasing to make it at least more coherent than the process that got me here.

The idea here is to nest all volunteer roles under a team, which allows a few things:

1. We can assign people as admins to a team and they get volunteer admin on all roles that team owns.
2. The role admin index is a bit more coherent
3. We don't have to prefix role names with their team now
4. (Not yet implemented) We can make the choose roles/schedule filters nest roles under their team, allowing people to quickly skim over stuff they don't want to go near.

I've also moved most of the volunteer related associations (role admin in particular) to the Volunteer model where they're closer to where they'll be used. We might want to reconsider Volunteer existing at all in the future but while it does lets use it.