Bring simple signup flow from the fizzy-saas gem

We skip the QB code and we fill external account ids automatically on creation with a sequence

See:
https://github.com/basecamp/fizzy-saas/pull/7

test/controllers/signup/completions_controller_test.rb

@@ -0,0 +1,43 @@
+require "test_helper"
+
+class Signup::CompletionsControllerTest < ActionDispatch::IntegrationTest
+  setup do
+    @signup = Signup.new(email_address: "newuser@example.com", full_name: "New User")
+
+    @signup.create_identity || raise("Failed to create identity")
+
+    sign_in_as @signup.identity
+  end
+
+  test "new" do
+    untenanted do
+      get new_signup_completion_path
+    end
+
+    assert_response :success
+  end
+
+  test "create" do
+    untenanted do
+      post signup_completion_path, params: {
+        signup: {
+          full_name: @signup.full_name
+        }
+      }
+    end
+
+    assert_response :redirect, "Valid params should redirect"
+  end
+
+  test "create with invalid params" do
+    untenanted do
+      post signup_completion_path, params: {
+        signup: {
+          full_name: ""
+        }
+      }
+    end
+
+    assert_response :unprocessable_entity, "Invalid params should return unprocessable entity"
+  end
+end