Base statistics on the number of users who interacted with the exploration, not the number who visited

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In the statistics tab we have a total number of users who have visited the 
exploration, which is used as the denominator for computations such as the 
completion rate, and the shading of explorations on the graph.

This proposal (made by Sean) is to change to use instead the number of users 
who have visited the exploration and submitted at least one answer. This is a 
more accurate reflection of the number of users who attempted the exploration, 
and so will act as a better basis for exploration statistics.

The first state will still show the number of people who visited the 
exploration as the incoming number. 

Original issue reported on code.google.com by jacobdav...@gmail.com on 3 Aug 2015 at 5:10