Add ability for board change balance

[SanderHageman]

Allow board members to top-up or remove (part of) the balance of users without it being registered as a transaction.
This might be needed in the case that someone (accidentally) buys something that isn't available physically, and they need their money back. Previously this could be done by labeling a transaction as having been done with cash and adding that money to their account but that option no longer exists.

Because these transactions currently need to be marked with either Pin, CC, or Mollie there will always be a discrepancy between the transactions with any of the providers and the transactions that mongoose thinks have taken place.

Alternatively: a way to refund a bought item to a user. This might be a better solution because it's less error-prone.