Provide a endpoint for safety handbook for form generators

This provides and endpoint for group admins to get the safety handbook
from so they can easily give a copy to their new starters. The endpoint
checks REMOTE_USER which is set by the use of Raven protection, so only
members of the department allowed to generate registration forms can
access it.

The form generation page is updated to warn if the handbook is not
available. Later we'll link the handbook here if it is available.