WebSearch: support for intbitset output format

* Adds support for a new output format in
  perform_request_search() function: the intbitset format.
  This format returns an intbitset representation of the
  result.  This means that the result is neither sorted nor
  ranked.  (closes #1460)

* Adds regression tests for of=intbitset.

* Also, fixes the fact that sorting no longer takes into
  account jrec and rg arguments, introduced in
  commit 0e6556031992aeb9229b722f4732b9550e8a28cb.

* Fixes typo in `get_tags_form_sort_fields` function name.

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