Scoreboard option for top-N only

[donnm]

It would be nice to have a top-N scoreboard where only the top submissions are displayed instead of the whole scoreboard. This could be implemented with a simple option topn in each scoreboard definition in course.yaml.

Possible implementation (I can submit a proper pull request when I have time):

diff --git a/inginious/frontend/webapp/plugins/scoreboard/__init__.py b/inginious/frontend/webapp/plugins/scoreboard/__init__.py
index 5c9005b3..0b5fdacb 100644
--- a/inginious/frontend/webapp/plugins/scoreboard/__init__.py
+++ b/inginious/frontend/webapp/plugins/scoreboard/__init__.py
@@ -53,6 +53,7 @@ class ScoreBoard(INGIniousAuthPage):
             scoreboard_name = scoreboards[scoreboardid]["name"]
             scoreboard_content = scoreboards[scoreboardid]["content"]
             scoreboard_reverse = bool(scoreboards[scoreboardid].get('reverse', False))
+            scoreboard_topn = int(scoreboards[scoreboardid].get('topn', 0))
         except:
             raise web.notfound()
 
@@ -121,12 +122,16 @@ class ScoreBoard(INGIniousAuthPage):
             solved = 0
             for taskid, coef in scoreboard_content.items():
                 if taskid in val:
+                    # This could be non-numeric
                     total += val[taskid]*coef
                     solved += 1
             overall_result_per_user[key] = {"total": total, "solved": solved}
         sorted_users = list(overall_result_per_user.keys())
         sorted_users = sorted(sorted_users, key=sort_func(overall_result_per_user, scoreboard_reverse))
 
+        if scoreboard_topn > 0:
+            sorted_users = sorted_users[0:scoreboard_topn]
+
         # Compute table
         table = []