Thalia Pay: Only allow revocation of mandates if all payments were processed

Job Doesburg
2020-03-09T17:59:50+01:00

Only allow revocation of mandates if all Thalia Pay payments were processed (the batch were all payments are in should be processed)

### Motivation

Because legally from the moment a mandate is revoked, we cannot use it anymore. And users of course cannot revoke their mandate if they still have open amounts (or actually they can, but then the treasurer should be notified of this and discuss with that person he/she should pay another way - I suggest we do not add that flow in the website).

### Desired functionality

When a user tries to revoke a mandate, but there are still Thalia Pay payments in batches that are not fully processed yet, the user should receive a message that he/she should contact the board by email to revoke the mandate because there are still open amounts to be paid.

### Suggested implementation