Integrate with PetScan v2 via its Referrer feature

[eggpi]

magnusmanske/petscan_rs#2

[eggpi]

#142 might require us to reconsider this: this issue is based on the assumption that the user always means to create a new PSID, whereas #142 is about a reasonable use case for importing an existing PSID.

This issue is not necessarily obsolete, but we want to make sure that use case is still possible, even though it may not be a primary case.