MatchingPairs: improve feedback after clearing the board

[BeritJanssen]

The "Final" page after clearing a board in MatchingPairs should provide additional information to participants:

• how hard was the board? Are the upcoming boards more or less challenging than the current one?
• how did participants perform, compared to other participants?
• what was their highest score so far?
• what was their average score?

[BeritJanssen]

To display how participants performed compared to others, we won't add a visual display just yet, but say "you perfomed better than X percent of games". This message should be suppressed if the participant's percentile is below 25%.

[BeritJanssen]

Ordering:

• This game
• Personal best
• Your average score
• Total score (i.e., over all sessions)

[BeritJanssen]

To implement: redesign MatchingPairs rules so that it's one board per session, then we can organize the successive boards as blocks attached to one experiment.

[jaburgoyne]

See also #518