Reduce SQL calls to obtain course_user's recent achievements in leaderboard view

[weiqingtoh]

Arising from #958 as well - since we need to get the 5 most recent achievement badges for each course_user, the current leaderboard view requires an SQL call for each course_user.

I suspect this might need us to use some manual preloader to do this, or maybe I'm wrong?

[lowjoel]

Yup, you'd probably need a manual preloader.

[ekowidianto]

Built-in preloading with scoped association with limit always returns empty records.
Requires has_many_through implementation for the manual preloader as there is currently only has_many implementation.
De-prioritising this as of now since the leaderboard page has no performance issue yet.