Don't publish the isAdmin property in the leaderboard publication.

Was published earlier just for admin users really, to filter out your own user
if you were an admin from the leaderboard. Fixed this another way now.
Added activeTo timestamp to problems and fixed retrieveAnswer with proper checking

activeTo should determine when it is allowed to attempt to answer a problem. If
that timestamp has passed, your time is up.

Previously it was possible to retrieve the answer to a problem you hadn't
solved.
Better date format for admins in problems list view
Reveal answer as button instead of link. Server throws nicer error when checking answers