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/application_controller.rb

@@ -37,7 +37,8 @@ class ApplicationController < ActionController::Base
   end
 
   def get_conferences
-    @conferences =Conference.all
+    domain = request.host
+    @conferences = Conference.get_conferences_to_list(domain, 0)
   end
 
   def current_ability