Allow setting admin=False on users
Allow setting admin=False on users

Fixes #2719

RafaelPalomar/girder,jbeezley/girder,manthey/girder,data-exp-lab/girder,jbeezley/girder,RafaelPalomar/girder,Kitware/girder,Xarthisius/girder,jbeezley/girder,girder/girder,Kitware/girder,manthey/girder,data-exp-lab/girder,girder/girder,kotfic/girder,kotfic/girder,Xarthisius/girder,RafaelPalomar/girder,girder/girder,kotfic/girder,data-exp-lab/girder,Xarthisius/girder,Kitware/girder,jbeezley/girder,kotfic/girder,Xarthisius/girder,data-exp-lab/girder,Kitware/girder,data-exp-lab/girder,RafaelPalomar/girder,manthey/girder,kotfic/girder,Xarthisius/girder,manthey/girder,girder/girder,RafaelPalomar/girder