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.

app/controllers/repp/v1/retained_domains_controller.rb

@@ -0,0 +1,11 @@
+module Repp
+  module V1
+    class RetainedDomainsController < ActionController::API
+      def index
+        domains = RetainedDomains.new
+
+        render json: { count: domains.count, domains: domains.to_jsonable }
+      end
+    end
+  end
+end