App provides safety handbook

The safety handbook can no longer be made available on our website without password protection (see RT 226276) and so this application must have access to a copy and be able to serve it up to people who request it and are authorized to see it. This branch adds a 'documents' directory to hold the handbook PDF and possibly other sensitive documents, and functionality to give the starter access to the handbook.

The real handbook can't be committed to this repository as it's public so has to be provided to the application at deploy time.