Register admin impersonation begin/end in audit log
Zendesk issue: https://gitlab.zendesk.com/agent/tickets/28711
Related to, but smaller in scope: #315
As an interim to #315, we should log when an admin begins and end impersonation. This will at least allow correlation between questionable user events and an admin impersonating a user. This protects the user from an abusive/rouge admin.
The hope is that the smaller scope of this issue would allow it to be done sooner than #315.
