Fix birthdays calendar button

Closes #782 (closed)

Previous behaviour

Steps to reproduce:

1. Click agenda (you see the calender view with a few events on it)
2. Click 'show birthdays'
3. Click list
4. There is nothing on most weeks, or sometimes only 1 event from that week
5. broken

New behaviour

Steps to validate that it works:

1. Click agenda (you see the calender view with a few events on it)
2. Click show verjaardagen
3. Click list
4. works fine