audit: add "authorization"

[yarikoptic]

The ongoing implementation in

• Add audit backend #1886

of current design concerns primarily with "mutating" operations. But another significant aspect is "authorized access". I wonder if we could add to audit trail

• notion of "new authorized session" which would record user id and datetime when such access happened
• new token regeneration
• unauthorized access attempts

(we might want to whitelist some here since I think we ATM use /api/users/me to get an idea on either user is already authorized, so it is kinda a legitimate use, and thus we have about 120 such each hour according to heroku logs)