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/serializers/rest/admin/domain_block_serializer.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class REST::Admin::DomainBlockSerializer < ActiveModel::Serializer
+  attributes :id, :domain, :created_at, :severity,
+             :reject_media, :reject_reports,
+             :private_comment, :public_comment, :obfuscate
+
+  def id
+    object.id.to_s
+  end
+end