BibRank: temporary file storage in CFG_TMPDIR

* Stores temporary data files used for plotting in CFG_TMPDIR
  instead of CFG_WEBDIR.
  Adds explicit 'bibrank_grapher_' prefix.
  This is not helping, but nor worsening the creation and
  servicing of graphs in load-balanced setups.

* Temporary directory set using 'dir' parameter instead of
  'tempfile.tempdir'.  (addresses #1594)

* Makes use of safer tempfile.mkstemp (instead of tempfile.mktemp).

Reviewed-by: Tibor Simko <tibor.simko@cern.ch>