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/system/admin_area/contacts_test.rb

@@ -0,0 +1,25 @@
+require 'test_helper'
+
+class AdminContactsTest < ApplicationSystemTestCase
+  def setup
+    super
+
+    @contact = contacts(:william)
+    sign_in users(:admin)
+  end
+
+  def test_display_list
+    visit admin_contacts_path
+
+    assert_text('william-001')
+    assert_text('william-002')
+    assert_text('acme-ltd-001')
+  end
+
+  def test_display_details
+    visit admin_contact_path(@contact)
+
+    assert_text('Street Main Street City New York Postcode 12345 ' \
+                'State New York Country United States of America')
+  end
+end