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,

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

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