Add configurable maximum to total user funds

[sekjal]

Is your feature request related to a problem? Please describe.
At this time, a user can transfer an unlimited amount of money to their Libki account. Depending on library policy, this money may or may not be refundable to users, which could lead to upset users who accidentally transferred more funds than intended.

Describe the solution you'd like
Add a configurable "Max Funds" setting, with an 'unlimited' default, that applies to funds on a user account any way they're added (staff edit, Stripe integration, Libki Print Station)

[sekjal]

If set to 0, this should prevent users from keeping any balance on their account at all, but not prevent them from paying per print job. Guest users should always have this 0 value logic enforced, since their accounts get wiped out at the end of the night.

This means that we'll have to briefly allow a balance (recording the credit) only if it's immediately followed by a debit of the same amount

[sekjal]

If set to 0, it should also be possible to pay for ALL pending jobs with a single credit transaction equaling the total, to save on transactions fees through e-commerce platforms