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

spec/controllers/api/v1/admin/domain_blocks_controller_spec.rb

@@ -0,0 +1,132 @@
+require 'rails_helper'
+
+RSpec.describe Api::V1::Admin::DomainBlocksController, type: :controller do
+  render_views
+
+  let(:role)   { 'admin' }
+  let(:user)   { Fabricate(:user, role: role) }
+  let(:scopes) { 'admin:read admin:write' }
+  let(:token)  { Fabricate(:accessible_access_token, resource_owner_id: user.id, scopes: scopes) }
+
+  before do
+    allow(controller).to receive(:doorkeeper_token) { token }
+  end
+
+  shared_examples 'forbidden for wrong scope' do |wrong_scope|
+    let(:scopes) { wrong_scope }
+
+    it 'returns http forbidden' do
+      expect(response).to have_http_status(403)
+    end
+  end
+
+  shared_examples 'forbidden for wrong role' do |wrong_role|
+    let(:role) { wrong_role }
+
+    it 'returns http forbidden' do
+      expect(response).to have_http_status(403)
+    end
+  end
+
+  describe 'GET #index' do
+    let!(:block) { Fabricate(:domain_block) }
+
+    before do
+      get :index
+    end
+
+    it_behaves_like 'forbidden for wrong scope', 'write:statuses'
+    it_behaves_like 'forbidden for wrong role', 'user'
+    it_behaves_like 'forbidden for wrong role', 'moderator'
+
+    it 'returns http success' do
+      expect(response).to have_http_status(200)
+    end
+
+    it 'returns the expected domain blocks' do
+      json = body_as_json
+      expect(json.length).to eq 1
+      expect(json[0][:id].to_i).to eq block.id
+    end
+  end
+
+  describe 'GET #show' do
+    let!(:block) { Fabricate(:domain_block) }
+
+    before do
+      get :show, params: { id: block.id }
+    end
+
+    it_behaves_like 'forbidden for wrong scope', 'write:statuses'
+    it_behaves_like 'forbidden for wrong role', 'user'
+    it_behaves_like 'forbidden for wrong role', 'moderator'
+
+    it 'returns http success' do
+      expect(response).to have_http_status(200)
+    end
+
+    it 'returns expected domain name' do
+      json = body_as_json
+      expect(json[:domain]).to eq block.domain
+    end
+  end
+
+  describe 'DELETE #destroy' do
+    let!(:block) { Fabricate(:domain_block) }
+
+    before do
+      delete :destroy, params: { id: block.id }
+    end
+
+    it_behaves_like 'forbidden for wrong scope', 'write:statuses'
+    it_behaves_like 'forbidden for wrong role', 'user'
+    it_behaves_like 'forbidden for wrong role', 'moderator'
+
+    it 'returns http success' do
+      expect(response).to have_http_status(200)
+    end
+
+    it 'deletes the block' do
+      expect(DomainBlock.find_by(id: block.id)).to be_nil
+    end
+  end
+
+  describe 'POST #create' do
+    let(:existing_block_domain) { 'example.com' }
+    let!(:block) { Fabricate(:domain_block, domain: existing_block_domain, severity: :suspend) }
+
+    before do
+      post :create, params: { domain: 'foo.bar.com', severity: :silence }
+    end
+
+    it_behaves_like 'forbidden for wrong scope', 'write:statuses'
+    it_behaves_like 'forbidden for wrong role', 'user'
+    it_behaves_like 'forbidden for wrong role', 'moderator'
+
+    it 'returns http success' do
+      expect(response).to have_http_status(200)
+    end
+
+    it 'returns expected domain name' do
+      json = body_as_json
+      expect(json[:domain]).to eq 'foo.bar.com'
+    end
+
+    it 'creates a domain block' do
+      expect(DomainBlock.find_by(domain: 'foo.bar.com')).to_not be_nil
+    end
+
+    context 'when a stricter domain block already exists' do
+      let(:existing_block_domain) { 'bar.com' }
+
+      it 'returns http unprocessable entity' do
+        expect(response).to have_http_status(422)
+      end
+
+      it 'renders existing domain block in error' do
+        json = body_as_json
+        expect(json[:existing_domain_block][:domain]).to eq existing_block_domain
+      end
+    end
+  end
+end