Log start and end of admin impersonation in audit log

As a first iteration on https://gitlab.com/gitlab-org/gitlab-ee/issues/315, we should show when an admin begins/ends impersonation of a user.

Proposal

Log an audit event for:

• An admin beginning impersonation of a user (and the user they are impersonating)
• An admin ending impersonation of a user (and the user they are impersonating)