Disable Thalia Pay payments for specific events

Is your feature request related to a problem? Please describe.

There might be events where Thalia Pay should not be available (study trip, events where the payment amount is variable like the orientation bbq, events where registration is not first come first serve like wine tasting or committee lunch)

Describe the solution you'd like

An option in the events admin that sets whether Thalia Pay is allowed for this event. The default should be true. When false, the payment button should not appear.

Probably we should also add something to the Payable interface about this so we can implement the same check in the process_payment code.

Motivation

Thalia Pay is quite critical and should work as intended always without too much manual hassle.

Describe alternatives you've considered

Manually removing payments that we do not want.

svthalia / concrexit