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.
Perhaps this:
Active Students
The count of students who interacted at least once by opening pages, playing videos, posting in discussions, submitting problems, or completing other activities. The date range includes all activities from midnight on the start date (inclusive) though midnight on the end date (exclusive).

(@mulby, you were right.)