Fix #2672 - Connect signed PuSH subscription requests to instance domain (#4205)

* Fix #2672 - Connect signed PuSH subscription requests to instance domain

Resolves #2739

* Fix return of locate_subscription

* Fix tests

db/migrate/20170714184731_add_domain_to_subscriptions.rb

@@ -0,0 +1,5 @@
+class AddDomainToSubscriptions < ActiveRecord::Migration[5.1]
+  def change
+    add_column :subscriptions, :domain, :string
+  end
+end