attach withdrawal route directly to a Team

[chadwhitacre]

Reticketing from #3399 (comment).

If Teams had a withdrawal route attached to them, that should dampen the confusion of having to create a ~user account in order to create a Team account: ~Gratipay is the owner of Gratipay, but ~whit537 (and possibly others) ought to be the manager.

On the other hand, Teams are surely going to want to give on Gratipay as well. Options:

1. They create a ~user account that is separate from the Team, and use that just for giving. It's not connected to the Team within Gratipay.
2. We enable Teams to give directly. This brings us back within sight of the old implementation of Teams as a particular kind of participant: they can do everything normal ~users can do, plus distribute payroll.

If we go for (2), then think about what a ~user can do that a Team can't: make voluntary payments to Teams, and take payroll from a Team. At some point we need to enable Teams of Teams, to enable growth beyond the Dunbar circle. Teams taking payroll from other Teams could address that.

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[chadwhitacre]

I'm dropping this from Pivot. This gets us into questions such as #3705 (comment) that I think we should postpone until post-Pivot:

Giving (to other teams)

We don't have this right now, and I'm not sure we should.