New PostgreSQL backup script

As well as getting rid of some rather broken shell script, this simplifies
things considerably by backing up the *whole* database. This means we can
provision a new machine with puppet and restore the entire database in one
shot.

The backup command we use which dumps *everything* :

- preserves OIDs (which isn't default!)
- creates users & databases
- trashes anything that's currently present (makes restore more
 reliable!)

See also https://github.gds/gds/pp-manual/pull/90

See https://www.pivotaltracker.com/story/show/66160244