Fixes #512 by filtering out the Administrator role from the role
dropdown and primary role options. The motivation here is that Inherited
Admins should be allowed to receive a secondary role specific to the
group they are being managed in, as they might lose their Admin
privileges at some point. Note that Inherited Admins will only be shown
on the list if they have an associated UserClassRole entry.

<img width="660" alt="image"
src="https://github.com/user-attachments/assets/1957628a-3a8c-4b65-925f-db516fa3cace">

<img width="715" alt="image"
src="https://github.com/user-attachments/assets/040bdfe6-229c-4375-bf40-be102d52af23">