Track challenge opening

[ColdHeat]

In theory, the dev version of CTFd means that every challenge load induces a hit to the server. This means that in theory we should be able to track each time a challenge opens.

Whether or not the db can take this kind of load increase I am not sure, but if it can, we should do it.

Perhaps this makes more sense in a cache versus a database.

[ColdHeat]

This also allows for the concept of tracking first views. The very first time that a user opens a challenge.

This is used by some people to achieve rudimentary challenge tracking but some people like the reports that are generated.

[ColdHeat]

Not doable for a 2.0.0 release. Also the metric would be very unreliable. A better idea would be to track when a user unlocks a challenge.