Make it clear when members are not allowed to go to events

Ghost User
2020-02-19T21:47:45+01:00

Make it clearer for members when they cannot go to events.

### Motivation
Sometimes members miss the email that tells them they are banned from events (because they forgot to pay their fines). This causes confusion when they cannot register for events.

### Desired functionality
Better information on the website when registrations are not allowed.

### Suggested implementation
A banner at the top of the site that clearly states when people are banned from something, and to contact the treasurer for more information.