Select the EventGroups from an Event queryset

This logic should be moved from this view into a custom manager
List the ongoing conferences
Refactored into Managers
Added event-group view, template, url
Link to event groups when appropriate
Updated event view to correctly thow Http404
Add related name for 'events' on an EventGroup
Prefetch and cache Events onto the EventGroup
Include ld+json events in EventGroup template
Only include if event has a group
Added link to event group on event page
Merge pull request #43 from ox-it/display-event-groups

Display event groups