Pull-down menu in Prepare Sequencing Runs page should only list PIs

Pulldown menu for PIs is slow, and pulls from the qiita.qiita_users list. Most people in this list won't be PIs. However, the PIs referenced in qiita.study point to email addresses in qiita.study_person that don't reference qiita.qiita_users addresses. Need to confirm that the PIs in this pulldown must be Qiita users before changing.