MatchingPairs: limit number of turns

[BeritJanssen]

After experiences with taking MatchingPairs out into the field, the research team thinks that there should be a stronger incentive to finish the game quickly, so participants don't resort to a brute force strategy. Therefore, they came up with the idea of a maximum number of turns: after 25 turns, the participant will be redirected to the feedback page, even if they didn't clear the board. The feedback will then be about if and how many turns they were under the maximum.

I advised to hold this change until after we have implemented other features in this milestone, as I think it will be time consuming to implement it: I expect this will go through several cycles of testing and improvements.