Perf: Instructor Dashboard performance regressions

[bmtcril]

In large scale load testing, the following Instructor Dashboard reports have poor performance, or are timing out:

• Currently enrolled learners per day
  • This only seems to be slow when no course filter is applied, we could possibly force a course filter here)
• Transcripts / Captions per video
  • This is pulling directly from xapi_events_all_parsed and should probably have an intermediate model of just these events with associated "enabled" / "disabled" values parsed out

[SoryRawyer]

The transcripts chart should be backed by a smaller MV instead of the larger xapi_events_all_parsed table, so performance should be improved.
I haven't been able to get much more performance out of the enrolled learners per day dataset, so I think requiring a course filter would be a reasonable change to enhance performance.

[bmtcril]

I think choosing a course is going to be required for v1 anyway 👍 Calling this one done, we'll do another round of testing once we have a v1 RC.