Add a SHARED_FILES_PATH option

This is to support sharing various files between multiple deployments
of Alaveteli.  This is quite specific to mySociety's deploy system at
the moment, in that the following files are still expected to be created
by the deployment system in each deployment:

  config/database.yml
  config/general.yml
  config/rails_env.rb
  config/newrelic.yml
  config/httpd.conf

The location of xapiandbs is managed by config/xapian.yml still.