Not-logged-in users become logged in with provisional user accounts

Requires challenge mode to be enabled
(static/js/common.js#challengeActive)
Provisional user account applied when not-logged-in user navigates to
one of the challenge pages (view crisis, view question or view answer),
allowing a new visitor to submit evidence, for example. Previous roles
still apply but new users can fit into the capabilities. Todo: Choose
username functionality/signup