Add balance based subscription/payment model

[braaar]

We want to add an optional feature to mulysa that can be toggled on or off based on the individual hacklab's preferences.

This is an alternate suscription/payment model where members use one personal reference for any payment to the hacklab. Incoming transactions from that member will be counted towards a balance, and active subscriptions and any other paid services will also count towards this balance. Members should be able to see their "debt" in their mulysa personal page and pay extra if they owe money, or hold off on payment if they have paid too much.

[braaar]

We should consider supporting an array of payment reference numbers, in case some members have had their reference number changed over time.

[braaar]

@artkaste could you check out the progress you have made so far in a PR so I can take a look at it? What were the challenges and roadblocks you faced?