Add minimum_credit parameter to ach_credit

This paves the way for paying out the full balance to a user when they
cancel their account.
Implement bank payout for cancelation

This is a second disbursement strategy alongside distributing as a final
gift.
Rough in UI for account cancelation; #54

Now that the three scripts involved in account cancelation have been
moved to (tested) methods on Participant, we're ready to wire up the UI.
This commit rebases the original work we did on the UI on top of the
separate line of development we ended up pursuing for the Participant
methods.
Rough in UI for case where there is no balance