User invitation form doesn't render the text of the newsletter checkbox in the contact permissions section

**Describe the bug** The user invitation form doesn't render the text of the `newsletter` checkbox in the "Contact permissions" section, and once submited raise an error. **To Reproduce** Steps to reproduce the behavior: 1. If someone has invited me, I'll receive an email with a link to complete/confirm my account 2. Click on the link in the email 3. Scroll down to "Contact permissions" 4. See a lonely checkbox **Expected behavior** It should show the explanatary text and once submited don't show an error **Extra data:** - Device: any - Device OS: any - Browser: any - Decidim Version: > 0.13 - Decidim installation: any