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/models/retained_domains.rb

@@ -0,0 +1,51 @@
+# Hiding the queries behind its own class will allow us to include disputed or
+# auctioned domains without meddling up with controller logic.
+class RetainedDomains
+  RESERVED = 'reserved'.freeze
+  BLOCKED = 'blocked'.freeze
+
+  attr_reader :domains
+
+  def initialize
+    @domains = gather_domains
+  end
+
+  def gather_domains
+    blocked_domains = BlockedDomain.order(name: :desc).all
+    reserved_domains = ReservedDomain.order(name: :desc).all
+
+    domains = blocked_domains.to_a.union(reserved_domains.to_a)
+
+    domains.sort_by(&:name)
+  end
+
+  def to_jsonable
+    domains.map { |el| domain_to_json(el) }
+  end
+
+  def domain_to_json(domain)
+    # Smelly, but ActiveRecord objects are weird and do not respond
+    # to usual syntax:
+    #   case a
+    #   when Array then "foo"
+    #   when Hash then "bar"
+    #   else "baz"
+    #   end
+    status = case domain.class.to_s
+             when 'ReservedDomain' then RESERVED
+             when 'BlockedDomain' then BLOCKED
+             end
+
+    punycode = SimpleIDN.to_ascii(domain.name)
+
+    {
+      name: domain.name,
+      status: status,
+      punycode_name: punycode
+    }
+  end
+
+  def count
+    domains.count
+  end
+end