Provide link to safety handbook from safety checklist form

The starter needs the safety handbook to complete their safety checklist
form. While they should have already been emailed a copy, they could
have lost it. This change adds an endpoint where a user with a valid token
can download the secret handbook, and updates the safety checklist
page to have a link to that. If the handbook hasn't been provided to the
app it tells the starter to go and ask their group admin instead.