log permission changes for audit reasons

[schiessle]

In security sensitive environments it is crucial to always know when access rights are granted and modified.

Therefore people should be notified on share / group folder permission change and change of group membership.

For audit reasons we should write to the audit log if:

• a user changed a share permission
• a user changed a group folder permission (advanced permissions)
• a group membership changed (a user was added/removed from a group)

Additionally it makes sense to inform users via a activity entries: #31448

[PVince81]

I've moved the suggestion about activities entries to #31448.
Implementing activities is much less straightforward than audit log entries, so let's keep those tasks separate.

[CarlSchwan]

a user changed a share permission

Already working

a user changed a group folder permission (advanced permissions)

Implemented in #31454 (bugfix) and nextcloud/groupfolders#1917

Should we maybe also add a log when a group folder is changed (no advanced permissions)?

a group membership changed (a user was added/removed from a group)

Already working