Pebbles security model

PSM comes in two parts. Visibility (Reading) and Actions (Create, Update, Delete):

• https://github.com/bengler/checkpoint/wiki/Pebbles-Security-Model-(Visibility)
• https://github.com/bengler/checkpoint/wiki/Pebbles-Security-Model-(Actions)

TLDR; Use access_group, access_group_membership and access_group_subtree to control read access. Use callback to control actions which modify content (create, update, delete).