WebSearch: always return full recID list via API

• Fixes API calls (of=id) when the search engine should always return the full hit list of record IDs. That is, the pagination arguments (rg, jrec) are ignored in this case, while sorting arguments (rm, sf) are respected.
  
  This used to be the long-standing behaviour of Invenio that was altered few months ago in the master branch as per the introduction of BibSort in 64fc501bfd4ed9b0f7b1762cc89c2c8389f5eb15. So this commit reverts back to the original behaviour as is present e.g. in the maint-1.0 branch.
  
  Note that this fixes one failed WebSearch regression test case.

• Adds more test cases and documentation to better cover this special paging behaviour in case of API output.

• Note: in the future we may want to alter the paging behaviour again, as part of introducing generators for Ajax like search engine calls, as part of the Flask branch.