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)

db/migrate/20170201193336_add_domain_to_conferences.rb

@@ -0,0 +1,6 @@
+class AddDomainToConferences < ActiveRecord::Migration
+  def change
+  	add_column :conferences, :domain, :string
+  	add_index :conferences, [:domain]
+  end
+end