Implement simplified commercials workflow

It's now possible to simply enter the url of the third party provider to enter a commercial.

db/migrate/20150417050953_add_url_to_commercial.rb

@@ -0,0 +1,11 @@
+class AddUrlToCommercial < ActiveRecord::Migration
+  class TempCommercial< ActiveRecord::Base
+    self.table_name = 'commercials'
+  end
+
+  def change
+    add_column :commercials, :url, :string
+
+    # Don't delete commercial_type and commercial_id for backward compatibility
+  end
+end

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: 20150415121038) do
+ActiveRecord::Schema.define(version: 20150417050953) do
 
   create_table "ahoy_events", force: true do |t|
     t.uuid     "visit_id"
@@ -79,6 +79,7 @@ ActiveRecord::Schema.define(version: 20150415121038) do
     t.string   "commercialable_type"
     t.datetime "created_at"
     t.datetime "updated_at"
+    t.string   "url"
   end
 
   create_table "conferences", force: true do |t|