[PERFORMANCE] [MER-4066] Dashboard optimizations

[darrensiegel]

This PR addresses a collection of performance issues in the Instructor Dashboard "Insights" tab, most prevalent in the "Practice Activities" and "Surveys" views.

There were two core performance issues:

1. The number of queries that needed to be run to display a collection of Practice or Survey activities (for a selected page) was proportional to the number of activities. I lost track of how many "per activity" queries were made, but I think it was around 3-5 per activity.

2. Some of the per activity queries and the non-per-activity queries run queried over the activity attempts table. For example, as a way to count the number of attempts. This is not scalable against production data.

The above two issues basically led to situations where the Practice Activities view was completely unusable for many courses in the system. You can see this right now on Proton by finding a decently large course, then trying to click on a practice page to see the activity details. The entire view just hangs.

The fix here was to re-implement the analytics gathering and preview rendering code to ensure that we:

1. minimize the total number of queries made when a page is selected
2. make 0 queries over any table in the attempt hierarchy
3. make 0 queries "per activity"

So in other words, we put a constant upper bound on the number of queries made when a page is selected.