Extract prolific url parameters automatically

[MarcKaufmann]

Prolific generates a URL with the following query parameters that we should extract:

?PROLIFIC_PID={{%PROLIFIC_PID%}}&STUDY_ID={{%STUDY_ID%}}&SESSION_ID={{%SESSION_ID%}}

Ideally this should be integrated with #44.

[Bogdanp]

Without having some way to ask Prolific to validate the SESSION_ID (via some sort of API, which doesn't seem to exist), automatically logging users in would be unsafe.

What might work is a workflow where we let users set a password the first time they visit one of these links and then subsequently they'll have to log in using that password (and the username/prolific email will be filled in for them). I think email verification should still be required to prevent someone from brute-force creating accounts and taking over ids.