Use 24h TTL for leaderboard data cache

LocMemCache doesn't evict unused entries, so infinite TTL would
keep inactive course data in memory forever. 24h TTL lets inactive
courses get cleaned up naturally while active courses stay cached
between weekly scoring runs.

Author: alexeygrigorev

data/views/leaderboard.py

@@ -27,7 +27,7 @@
 
 logger = logging.getLogger(__name__)
 
-LEADERBOARD_DATA_CACHE_TTL = None  # Cache forever, invalidated by update_leaderboard()
+LEADERBOARD_DATA_CACHE_TTL = 86400  # 24 hours; also invalidated by update_leaderboard()
 
 
 def _build_leaderboard_data(course):