use django-account-balances for accounting

[yuwash]

Currently this uses empty EmptyPayment for main accounting (no payment possible) and InfinitePayment for bonus (you can wash indefinitely for free).

We should use django-account-balances as Netz-AG proposes, however, the version on the repository carver/django-account-balances is outdated and incompatible with both Python 3 and Django 1.11. We should instead use the private fork hosted by Netz-AG (link works only in the house network). There’s also the upstream django-oscar/django-oscar-accounts project that’s very active, however which is still dependent on oscar.

[yuwash]

I just started implementing this in the "accounts" branch.

One unsolved issue is this exclude call that shouldn’t be necessary, but the test "test_bonuspayment" currently fails if I remove this. Somehow it finds multiple account instances for the waschexample user.