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/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20160815140302) do
+ActiveRecord::Schema.define(version: 20170201193336) do
 
   create_table "ahoy_events", force: :cascade do |t|
     t.uuid     "visit_id",   limit: 16
@@ -98,8 +98,11 @@ ActiveRecord::Schema.define(version: 20160815140302) do
     t.text     "description"
     t.integer  "registration_limit", default: 0
     t.string   "picture"
+    t.string   "domain"
   end
 
+  add_index "conferences", ["domain"], name: "index_conferences_on_domain"
+
   create_table "conferences_questions", id: false, force: :cascade do |t|
     t.integer "conference_id"
     t.integer "question_id"