Improve clarity for attendance selection button (especially on mobile page)

[ergoen]

Hi,

Om the mobile UI (and to some extent the desktop UI also), the "question mark" button to select attendance/non-attendance is very discreet currently. For events with a single date, it is very easy to completely miss the option entirely resulting in users not selecting anything and showing up as not-attending on the list.

Suggested alternatives:

• Only allow clicking "save" if the user has selected at least one date as explicitly attending/not attending.
• (For single date events this is by far the most user-friendly option) Set the default option to attending.