#2422 add support for deep links bypassing password entry

[RichardBradley]

See #2422

We have UR where users are transitioning from desktop to mobile within our prototypes, by scanning a QR code.

Some of our users have been confused by the password entry page appearing during that transition.

This PR implements #2422 by allowing links to include the prototype password in the query string.

We have updated our QR codes to do that, meaning that users who follow our prototype journey will no longer see the password entry page part way through their journey.

As I noted in the code comments:

      // Care should be taken to avoid leaking URLs containing the password where
      // web crawlers like Google might find them, because if Google indexes the prototype
      // site then it can get flagged as a gov.uk impersonating phishing site and
      // globally blocked.

If this is accepted, the docs will need updating. Where is the code for that?