WebComment: attachments in multi-node setup

* Support file attachments in environments using several workers,
  by (a) temporarily storing files in CFG_TMPSHARED, and (b) moving
  files to final storage area using shellutils.move() instead of
  os.renames().

Signed-off-by: Jerome Caffaro <jerome.caffaro@cern.ch>
Tested-by: Tibor Simko <tibor.simko@cern.ch>