Remove mySociety log handling from install scripts

The deployment system should deal with putting logs in the desired
place. As this script gets run on a variety of systems it can cause
problems for non-mySociety reinstallers.
Update log paths

Use the standard rails locations. Deployment systems should move the log
directory.
Persist log directory across capistrano deployments
Add log directory to shared directories

Now that the install script leaves the log directory alone (a5f79a5) we
let the generic mechanism for SHARED_DIRECTORIES handle creating a
persisted log directory and symlinking it together.
Add log/ to switch-to-shared script

script/rails-deploy-before-down will complain if log is listed in the
SHARED_DIRECTORIES configuration variable and already exists in
APP_ROOT. It will recommend this script be used to migrate to a
timestamped deployment structure, so adding log/ makes the transition of
the log directory easier.
Add upgrade notes for new shared log configuration