Adds domain support to OSEM

- This is a very basic support. All it affects is the listings
  on the homepage. Every conference can set a domain, and as
  long as the request host matches this domain, the conference
  will be shown on the list
- If a conference does not have a domain set (which will be the
  case for older conferences), such conferences are shown
  always
- Conference creation and edits now have an optional domain field
- Also adds corresponding migrations (+index)

app/controllers/admin/conferences_controller.rb

@@ -176,7 +176,7 @@ module Admin
     private
 
     def conference_params
-      params.require(:conference).permit(:title, :short_title, :description, :timezone,
+      params.require(:conference).permit(:title, :short_title, :description, :timezone, :domain,
                                          :start_date, :end_date, :rooms_attributes, :tracks_attributes,
                                          :tickets_attributes, :event_types_attributes,
                                          :picture, :picture_cache, :questions_attributes,