ls_export() should join responses to participants, not vice versa

[netique]

In case a questionnaire has a participant table and no responses, ls_export() produces a somewhat misleading output with quite naturally lists all participants with no responses, but that might appear as the participants posted blank responses and we have a large chunk of missing data. The proposal is to join right_join(participants, responses) and to use ls_participants() and ls_responses() for other purposes.