Add the SHARED_FILES_PATH, SHARED_FILES & SHARED_DIRECTORIES options

This is to support sharing various files between multiple deployments
of Alaveteli.  If you have set SHARED_FILES_PATH, SHARED_FILES and
SHARED_DIRECTORIES then the rails-deploy-before-down script will ensure
that a symlink is created from each of the files or directories
mentioned in SHARED_FILES and SHARED_DIRECTORIES to a corresponding
path within SHARED_FILES_PATH.
Add a script to switch mySociety deployments to using shared files

This is required by our switch to timestamped-based deploys.  There
are certain files and directories (e.g. cache, the Xapian databases,
generated graphs) that should be shared between deployments,
rather than recreating them when creating the next deployment in
a new directory.

This script should be run as a one-off before switching an
instance to using timestamped deploys, to ensure that the existing
data is stored in the shared subdirectory of the vhost directory,
so that any future deployment can simply create symlinks into the
shared directory.