Move user db creation to service

app/controllers/signup_controller.rb

@@ -94,14 +94,15 @@ class SignupController < ApplicationController
   end
 
   def complete_signup
-    @user.save!
     session[:new_user] = nil
     session[:validation_error] = nil
 
     CreateAccount.call(
       username: @user.cn,
+      domain: "kosmos.org",
       email: @user.email,
-      password: @user.password
+      password: @user.password,
+      invitation: @invitation
     )
 
     @invitation.update! invited_user_id: @user.id, used_at: DateTime.now