FIX: can_admin_group should be true when creating a new group. (PR #13671)

It looks like this regressed in #10432.

A user can create a group if they’re an admin or if they’re a mod and the “moderators_manage_categories_and_groups” setting is enabled, so it’s safe to always set “can_admin_group” to true for new groups.

It will let us configure automatic membership, default title, and effects on create.