Add instance configuration options
This adds the general possibility of configuring instances and implements the two following configuration options:
- Choosing whether the instance should appear in the public list of instances (cf. #19 (closed)).
- Option of requiring everyone to mark themselves as ready before displaying results and locking them for a specified amount of time (cf. #3 (closed)).
Along the way I also changed the stack resolver to LTS 16.1 because I needed a newer version of time
.
I'm not entirely satisfied with the UI and the locking mechanism, so if you have any suggestions for those that would be especially nice.