privthreatmodel notes and summary

Notes are in
https://github.com/w3c/ping/blob/master/summaries/breakout-privacythreat-20190917.md
in
addition to the IRC log at
https://www.w3.org/2019/09/18-privthreatmodel-minutes.html.

We found broad consensus to build a target privacy threat model in PING,
starting from https://jyasskin.github.io/privacy-threat-model/. We also got
several concrete suggestions for things to ensure that document covers,
which are recorded in the notes.

Jeffrey