data-mkt-user can be true/false/falsey (bug 1107700)

We're only going to show the logout interstitial at the start of the payment flow (e.g. there's a jwt) and we know that the user didn't come from marketplace.

This defaults the `data-mkt-user` to an empty string (which essentially means we don't know whether the user came from marketplace or not.)

If there's a jwt and no email  then we know the user definitely didn't come from marketplace so the value is `'false'`. 

When the jwt is present and has the email the value is `'true'`

This way it feels like the data-mkt-user value is more correct rather than only being `'true'` or `'false'`. Previously as we defaulted to false this could be false when the user comes to a different view despite actually having originated the flow from the marketplace.