Change get(userID) to getLoggedInContactID()

Just logging this to track some code tidy up. Mostly replacing

$session = CRM_Core_Session::singleton();
$userID = $session->get('userID');

With
$userID = CRM_Core_Session::singleton()->getLoggedInContactID();

which is just a minor code simplification & my main motive is improve the code created when people do copy & paste
https://github.com/civicrm/civicrm-core/pull/8635