User questionnaire before user logs out

[BeritJanssen]

When the user logs out, first present an (optional) questionnaire about their user experience

• get input for questionnaire from consortium
• make questionnaire form
• add form to interface

[jgonggrijp]

But preferably just reuse the existing feedback form.

Note: not every time, just once in about every 20 logouts.
Note: need to take into account that logged-out users might not be able to submit the feedback form.