* 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
