Simple implementation of retained domains API endpoint

This needed a new name, there are several classes of database object
to be included in that endpoint.

Currently, there is one list ordered by name, with each object
containing status and ascii name for convenience. Can be converted to
multiple fields (reserved and blocked separately). Also contains total
count.

Includes CORS preflight which seems to be a known problem for Rails in
the past.

test/integration/repp/retained_domains_test.rb

@@ -0,0 +1,39 @@
+require 'test_helper'
+
+class ReppV1RetainedDomainsTest < ActionDispatch::IntegrationTest
+  # Uses magical fixtures, will fail once fixtures inside are changed:
+  # test/fixtures/blocked_domains.yml
+  # test/fixtures/reserved_domains.yml
+
+  def test_get_index
+    get repp_v1_retained_domains_path
+    response_json = JSON.parse(response.body, symbolize_names: true)
+
+    assert response_json[:count] == 3
+
+    expected_objects = [{ name: 'blocked.test',
+                          status: 'blocked',
+                          punycode_name: 'blocked.test' },
+                        { name: 'blockedäöüõ.test',
+                          status: 'blocked',
+                          punycode_name: 'xn--blocked-cxa7mj0e.test' },
+                        { name: 'reserved.test',
+                          status: 'reserved',
+                          punycode_name: 'reserved.test' }]
+
+    assert_equal response_json[:domains], expected_objects
+  end
+
+  def test_cors_preflight
+    process :options, repp_v1_retained_domains_path, headers: { 'Origin' => 'https://example.com' }
+
+    assert_equal('https://example.com', response.headers['Access-Control-Allow-Origin'])
+    assert_equal('POST, GET, PUT, PATCH, DELETE, OPTIONS',
+                 response.headers['Access-Control-Allow-Methods'])
+    assert_equal('Origin, Content-Type, Accept, Authorization, Token, Auth-Token, Email, ' \
+                 'X-User-Token, X-User-Email',
+               response.headers['Access-Control-Allow-Headers'])
+    assert_equal('3600', response.headers['Access-Control-Max-Age'])
+    assert_equal('', response.body)
+  end
+end