Add confirmation message to events registration cancellation

Ghost User
2018-03-20T17:59:40+01:00

Closes #590

### Short description

### Steps to use this feature

1. Create an event with registration
2. Register
3. Cancel within the deadline

Closes #590 (closed)

Short description

Steps to use this feature

1. Create an event with registration

2. Register

3. Cancel within the deadline