Allow users to configure Discoto's behavior on a per-topic basis

I think for now an embedded YAML block in the issuable description would work well, something like:

Discoto Configuration

summary:
  max_items: -1 # -1 means no limit
  sort_by: date # one of date, activity, outcome
  sort_direction: ascending # one of ascending, descending  

See the discoto documentation for more details