Add /api/v1/admin/domain_blocks (#18247)

* Add /api/v1/admin/domain_blocks

Fixes #18140

- `GET /api/v1/admin/domain_blocks` lists domain blocks
- `GET /api/v1/admin/domain_blocks/:id` shows one by ID
- `DELETE /api/v1/admin/domain_blocks/:id` deletes a given domain block
- `POST /api/v1/admin/domain_blocks` to create a new domain block:
  if it conflicts with an existing one, returns an error with
  an attribute `existing_domain_block` with the rendered domain block

* Simplify conflict handling as suggested in review

app/models/domain_block.rb

@@ -16,6 +16,7 @@
 #
 
 class DomainBlock < ApplicationRecord
+  include Paginable
   include DomainNormalizable
   include DomainMaterializable