Allow subscription-based payments

[dictvm]

I'd like to include coffee-consumption into the mete-system. However, paying each cup of coffee (e.g. 5cents per cup) via mete is cumbersome and allows for Verpeilung, e.g. forgotten payments. I prefer to allow people to subscribe to a coffee-flatrate for a fixed sum, which should be customisable but default to 5EUR.

As a user, I'd expect the interface to enable an additional option in my user's edit-page where there should be a binary subscription-toggle and an edit-form to select the sum. Payment should be done on a monthly basis as long as the user's balance allows it and isn't negative. In case of a negative balance the user's page should display a red warning that the subscription couldn't be paid for, just like the orange warning does, when a user pays for a drink without sufficient funds.