Build out admin UI

[tkkuehn]

The problem

We have user logins now, but no way for admins to review user submissions as intended. It would be good to provide a page for "admin"-level users to look at fiducial placements users have submitted and collect statistics about those placements.

[kaitj]

The flask-admin library should be able to help with this. Can build out custom views to use with the library.
https://flask-admin.readthedocs.io/en/latest/

flask-security may also be helpful, but not sure if it is necessary.