[#823 #884 #905] Approve users and updated registration process

- Styling on 'registration complete' page fixed
- Error pages working
- Approve users on user management page
  - A green check is shown if an employment is not approved yet (click to approve)
  - A red cross is shown if an employment is approved (click to delete)
  - Nothing is shown if the -- logged in -- user is not connected to the organisation in the employment
- User management link not visible to everyone