Allow adding custom buttons to dashboard

Introduce a model to specify further custom buttons that will be displayed in the dashboard for a given event.

added feature label

assigned to @bhaettasch

mentioned in commit 2f0936df

mentioned in merge request !9 (merged)

added work-in-progress label

closed via merge request !9 (merged)

mentioned in commit 63f87294

mentioned in commit fe24cd4e