small refactor of volunteer user creation

- code slimmer
- dont send invite_sent notification in case user creation and invitation fails

app/controllers/volunteers_controller.rb

@@ -45,11 +45,11 @@ class VolunteersController < ApplicationController
   private
 
   def invite_volunteer_user
-    new_user = User.new(email: @volunteer.contact.contact_emails.first.body,
-      password: Devise.friendly_token, role: 'volunteer')
-    new_user.save
-    @volunteer.user = new_user
-    new_user.invite!
+    new_user = User.new(
+      email: @volunteer.email, password: Devise.friendly_token,
+      role: 'volunteer', volunteer: @volunteer
+    )
+    return unless new_user.save && new_user.invite!
     redirect_to volunteers_path, notice: t('invite_sent', email: new_user.email)
   end