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.
Is there any way to surface the actual date range? If not, I'd suggest giving more guidance about what is meant by "the last week". For example, it's useful to know whether this is for the last complete Sun-Sat time frame, or for the  7 days through when the instructor dashboard was loaded/refreshed... Let me know.
Add period at end of sentence.