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
Tag -> Topic
Added link to event group on event page
https talks-dev
Create missing topics into db
Merge pull request #43 from ox-it/display-event-groups

Display event groups
Merge branch 'master' into topics-suggestions

* master:
  Added link to event group on event page
  Only include if event has a group
  Include ld+json events in EventGroup template
  Prefetch and cache Events onto the EventGroup
  Add related name for 'events' on an EventGroup
  Updated event view to correctly thow Http404
  Link to event groups when appropriate
  Added event-group view, template, url
  Refactored into Managers
  List the ongoing conferences
  Select the EventGroups from an Event queryset
Topics suggestion URL configuration