Invalidate exchange routes during account close

[chadwhitacre]

For whatever reason we don't seem to have implemented removing payment methods at Balanced during account close. It appears that this is in fact possible now:

balanced/balanced-api#22
balanced/balanced-api#29

So we should go ahead and do that during account close. The end result is that we still have a Balanced account for each relevant Gratipay account, and the old payment methods are still there, but the sensitive financial data is pruned (we retain last four digits only).

Reticketed from #397 (comment).

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[chadwhitacre]

As part of implementing this we should go through and remove all payment instruments relating to currently closed accounts.