Fix undefined LetsEncrypt Terms of Service

Restore the method which was deleted on later stages of develpment
+ specs

changelogs/unreleased/33230-undefined-method-terms_of_service_url.yml

+---
+title: Fix 500 error on clicking to LetsEncrypt Terms of Service
+merge_request: 18263
+author:
+type: fixed

lib/gitlab/lets_encrypt.rb

@@ -5,5 +5,9 @@ module Gitlab
     def self.enabled?
       Gitlab::CurrentSettings.lets_encrypt_terms_of_service_accepted
     end
+
+    def self.terms_of_service_url
+      ::Gitlab::LetsEncrypt::Client.new.terms_of_service_url
+    end
   end
 end

spec/controllers/admin/application_settings_controller_spec.rb

@@ -143,4 +143,22 @@ describe Admin::ApplicationSettingsController do
       expect(response).to redirect_to(admin_runners_path)
     end
   end
+
+  describe 'GET #lets_encrypt_terms_of_service' do
+    include LetsEncryptHelpers
+
+    before do
+      sign_in(admin)
+
+      stub_lets_encrypt_client
+    end
+
+    subject { get :lets_encrypt_terms_of_service }
+
+    it 'redirects the user to the terms of service page' do
+      subject
+
+      expect(response).to redirect_to("https://letsencrypt.org/documents/LE-SA-v1.2-November-15-2017.pdf")
+    end
+  end
 end

spec/lib/gitlab/lets_encrypt_spec.rb

@@ -24,4 +24,16 @@ describe ::Gitlab::LetsEncrypt do
       it { is_expected.to eq(false) }
     end
   end
+
+  describe '.terms_of_service_url' do
+    before do
+      stub_lets_encrypt_client
+    end
+
+    subject { described_class.terms_of_service_url }
+
+    it 'returns the url' do
+      is_expected.to eq("https://letsencrypt.org/documents/LE-SA-v1.2-November-15-2017.pdf")
+    end
+  end
 end