Change test structure to follow closer newer Rails 5 rules

* Create new class called ApplicationIntegrationTest, so we don't have
  to override ActionDispatch::IntegrationTest
* Move UI tests to inherit from ApplicationSystemTestCase
* Existing REST API or EPP tests inherit from
  ApplicationIntegrationTest.
* Move `require 'application_system_test_case'` at the end of
  `test_helper`

I don't particularly agree with the Rails' convention of treating UI
tests as system tests and API tests as integration tests, but I see no
benefit in actively fighting against it.

test/integration/api/domain_contacts_test.rb

@@ -1,6 +1,6 @@
 require 'test_helper'
 
-class APIDomainContactsTest < ActionDispatch::IntegrationTest
+class APIDomainContactsTest < ApplicationIntegrationTest
   def test_replace_all_tech_contacts_of_the_current_registrar
     patch '/repp/v1/domains/contacts', { current_contact_id: 'william-001',
                                          new_contact_id: 'john-001' },

test/integration/api/domain_transfers_test.rb

@@ -1,6 +1,6 @@
 require 'test_helper'
 
-class APIDomainTransfersTest < ActionDispatch::IntegrationTest
+class APIDomainTransfersTest < ApplicationIntegrationTest
   setup do
     @domain = domains(:shop)
     @new_registrar = registrars(:goodnames)

test/integration/api/nameservers/put_test.rb

@@ -1,6 +1,6 @@
 require 'test_helper'
 
-class APINameserversPutTest < ActionDispatch::IntegrationTest
+class APINameserversPutTest < ApplicationIntegrationTest
   def test_replaces_registrar_nameservers
     old_nameserver_ids = [nameservers(:shop_ns1).id,
                           nameservers(:airport_ns1).id,