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/conferences_controller.rb

@@ -5,7 +5,8 @@ class ConferencesController < ApplicationController
   load_resource :program, through: :conference, singleton: true, except: :index
 
   def index
-    @current = Conference.where('end_date >= ?', Date.current).reorder(start_date: :asc)
+    domain = request.host
+    @current = Conference.get_conferences_to_list domain
     @antiquated = @conferences - @current
   end