Support user sessions going in/out of PetScan from other tools

[eggpi]

As discussed in Wikimania, Citation Hunt has a flow where users click a link to PetScan, come up with a query, and are then supposed to navigate back to the same CH tab and paste the ID of the resulting PS query.

It would be neat if we could integrate the two tools a little more closely. For example, CH would have a link to:

https://petscan.wmflabs.org?language=en&depth=5&toolname=Citation%20Hunt&toolurl=<some opaque url>

And PS would have a link for returning to the tool, which, when clicked, sends the user to:

https://<some opaque url>?psid=123456

when they are done with their query.

[magnusmanske]

Implemented with ce14e78 .

See documentation at https://meta.wikimedia.org/wiki/PetScan#Referrer