Add a Do Not Track policy

This will prevent Persona from being blocked by privacy-protecting
add-ons such as the EFF's Privacy Badger.

https://www.eff.org/privacybadger#how_to_comply_with_dnt

The temporary policy is copied verbatim from https://www.eff.org/files/dnt-policy-preliminary.txt

https://github.com/EFForg/privacybadgerfirefox/issues/98
Merge pull request #4129 from fmarier/dnt-policy

Add a Do Not Track policy