Refactored ParticipantListing.php

1) Commuted assigns in if statements into assigning, then checking, as otherwise fails code validators and breaches CiviCRM coding conventions.

2) Refactored repeated code found while repairing those errors into a separate reusable function that accepts parameters. I only did this where the code was identical. Where there was any distinction I didn't do this to prevent introducing bugs.

https://issues.civicrm.org/jira/browse/CRM-14434