Get active student count from new Analytics DATA API

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@mulby @clintonb minor changes to be more consistent with the naming. Also updated the tooltip display per @lamagnifica lastest draft.

The requirements depends on making a first release to the data client repository.
I was actually thinking in just using `request` for now, and only pull in `analyticsclient` later when it gets more stable, what do you thing?

I'll move the imports.