AdminUI - don't show or allow deleting reserved custom fields/groups
Partly a followup to #4338 (closed)
- It shows you the reserved groups in the listing - regular core doesn't do this, 'cuz reasons.
- It lets you delete fields in those groups - not good.
- It lets you delete the group after you've deleted those fields - not good.
Reserved custom groups are always created by extensions, and often depend on them being unchangeable by users.