Fixed bug due to Django 1.6 update. The get_or_create call in refresh_users_from_ldap.py needed some default arguments. Otherwise, it tries to create a user with an empty username, and that will collide with another user with an empty filename and cause an unhandled exception.

Added Bayestar MIC robot DN and user. This is a temporary robot for ER5. Should probably be removed when Leo doesn't need it anymore.

Added migrations for gdb_pe robot cert and 'background' tag. Also added 'background' to the list of blessed tags in the settings module.

I (Branson) found this local change to refresh_users_from_ldap on archie. It looks harmless, so I'm keeping it.

https://bugs.ligo.org/redmine/issues/732