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

config/routes.rb

@@ -570,6 +570,7 @@ Rails.application.routes.draw do
           end
         end
 
+        resources :domain_allows, only: [:index, :show, :create, :destroy]
         resources :domain_blocks, only: [:index, :show, :update, :create, :destroy]
 
         namespace :trends do