Change to re-add "Potential Consent"

This is the text that was previously in the TPE document, plus the bolded
addition from Justin Brookrman:

5.2.7 Potential Consent (P)

A tracking status value of P means that the origin server does not know, in
real-time, whether it has received prior consent for tracking this user,
user agent, or device, but promises not to use or share any DNT:1 data
until such consent has been determined, and further promises to delete or
de-identify within forty-eight hours any DNT:1 data received for which such
consent has not been received.

Since this status value does not itself indicate whether a specific request
is tracked, an origin server that sends a P tracking status value
*must* provide
an edit<http://www.w3.org/2011/tracking-protection/drafts/tracking-dnt.html#dfn-edit>
member
in the corresponding tracking status representation that links to a
resource for obtaining consent status.

The P tracking status value is specifically meant to address audience
survey systems for which determining consent at the time of a request is
either impractical, due to legacy systems not being able to keep up with
Web traffic, or potentially "gamed" by first party sites if they can
determine which of their users have consented. The data cannot be used for
the sake of personalization. If consent can be determined at the time of a
request, the C<http://www.w3.org/2011/tracking-protection/drafts/tracking-dnt.html#dfn-c>tracking
status is preferred. *If an origin server subsequently determines that it
does not have prior consent to track a user, the origin server may not then
disregard the user's DNT:1 signal; rejections of DNT:1 signals must be made
in real-time, using the tracking status value of D defined in 5.2.8.*

--ronan