Don't delete panel applicants on attendee delete

I'm sick of manually handling duplicate registrations that only exist because we created a new badge for a panel. This change will let me just delete the extra attendee and re-associate the panelist to the existing attendee.

uber/models/panels.py

@@ -210,7 +210,7 @@ def has_been_accepted(self):
 
 class PanelApplicant(SocialMediaMixin, MagModel):
     app_id = Column(UUID, ForeignKey('panel_application.id', ondelete='cascade'))
-    attendee_id = Column(UUID, ForeignKey('attendee.id', ondelete='cascade'), nullable=True)
+    attendee_id = Column(UUID, ForeignKey('attendee.id', ondelete='SET NULL'), nullable=True)
     submitter = Column(Boolean, default=False)
     first_name = Column(UnicodeText)
     last_name = Column(UnicodeText)