Disable payment button for Thalia Pay in payment widget

Ghost User
2020-02-19T21:48:25+01:00

Closes #980

#### Previous behaviour
Steps to reproduce:

1. One can click the process button for Thalia Pay for a membership registration
2. It crashes

#### New behaviour
Steps to validate that it works:

1.Cannot click it anymore

Closes #980 (closed)

Previous behaviour

Steps to reproduce:

1. One can click the process button for Thalia Pay for a membership registration

2. It crashes

New behaviour

Steps to validate that it works:

1.Cannot click it anymore