Switch from /groups to /groups-backup. Filesystem meltdown!

Orchestra filesystem has melted down.  A read-only backup is available
at /groups-backup.  Switch to that.  Also change location of system
python the /home, since it is still available.  Finally, since
/groups-backup is not actually available from the web servers, only the
login servers (WTF?) put the site in "maintenance mode" for the time
being.

Oh, and replace diabric dependency with fabvenv.  Also, add a .pth file
to the virtualenv pointing to the app/code dir.