Add /api/v1/admin/domain_allows (#18668)

- `GET /api/v1/admin/domain_allows` lists allowed domains
- `GET /api/v1/admin/domain_allows/:id` shows one by ID
- `DELETE /api/v1/admin/domain_allows/:id` deletes a given domain from the list
  of allowed domains
- `POST /api/v1/admin/domain_allows` to allow a new domain:
  if that domain is already allowed, the existing DomainAllow will be returned

app/models/domain_allow.rb

@@ -11,6 +11,7 @@
 #
 
 class DomainAllow < ApplicationRecord
+  include Paginable
   include DomainNormalizable
   include DomainMaterializable