Add a Config entry `view_challenges_unregistered` to indicate whether
unregistered users can view challenges. Add the setting to the admin config
page.
Add can_view_challenges() to utils to test if a user is either authed, or the
configuration allow unauthenticated users to view the challenges.
Return a HTTP 401 Unauthorized error when the /chals/solves API can't provide
results for an unauthenticated user. This is needed because the client side
code in `chalboard.js` doesn't know if it's logged in or not and requests this
anyway. (And AJAX doesn't handle redirects very well.) Alternately the client
could actually know if they're logged in and not make needless API calls.
When an unregistered user attempts to submit a flag, it will also fail. The
user will be redirected to a login page.
