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

config/locales/en.yml

@@ -428,6 +428,7 @@ en:
       destroyed_msg: Domain block has been undone
       domain: Domain
       edit: Edit domain block
+      existing_domain_block: You have already imposed stricter limits on %{name}.
       existing_domain_block_html: You have already imposed stricter limits on %{name}, you need to <a href="%{unblock_url}">unblock it</a> first.
       new:
         create: Create block