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/routes.rb

@@ -559,6 +559,8 @@ Rails.application.routes.draw do
           end
         end
 
+        resources :domain_blocks, only: [:index, :show, :update, :create, :destroy]
+
         namespace :trends do
           resources :tags, only: [:index]
           resources :links, only: [:index]