FIX: Change is_staff to is_admin to match other places where guardian.allowed_category_ids is used
https://meta.discourse.org/t/security-permissions-and-messages-displayed-on-group-url/22169/17
Merge pull request #2971 from cpradio/pr-fix-allowed-category-ids-for-group-posts

FIX: Change is_staff to is_admin for group posts to match other places