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>