uc_signup_admin_list should use VBO, filter out anyonmous

Issue Summary

The administrative signup list that uc_signup provides should be a VBO view like signup_vbo_admin_list, since it's reasonable to assume that folks managing signup either want to mark attendees as attended, did not attend, or cancel their signups.

I'm ok making uc_signup dependent on VBO.

We should filter out signups from anonymous users since COD assumes that all attendees have accounts, and uses anonymous signups as placeholders which should not be visible on the general attendee list.