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>