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

spec/controllers/api/push_controller_spec.rb

@@ -21,6 +21,7 @@ RSpec.describe Api::PushController, type: :controller do
           'https://callback.host/api',
           'as1234df',
           '3600',
+          nil
         )
         expect(response).to have_http_status(:success)
       end