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
Merge pull request #276 from alphagov/update-postgres-backup-script

New PostgreSQL backup script