Audit: add public audit to router

Create a specific audit for channel owners/admins to have access to. Path should be external/notifications/{notification_id}.

Should be very limited and contain only events and counts (not listed users)

• Start processing
• Unsubscribed users: total count
• If targeted: target users and groups list, (not expanded targets)
• If targeted and intersection: list of users and groups, not expanded members
• Expanded recipients, total count