fix(isLegitimatedForUserId): Setup mountpoints to check file access

fix(isLegitimatedForUserId): Setup mountpoints to check file access

This fixes workflows on groupfolders, as it will consider access to
files in groupfolders.

It also fixes false positives where access to files was limited by other
means not taken into account before, e.g. access control.

For postDelete events, check for permissions of the parent folder
instead, as the file itself no longer exists.

Fixes: nextcloud/flow_notifications#71
Signed-off-by: Jonas <jonas@freesources.org>