User activity by day and month

Early review of direction, here's the approach:

* Refactor user activity to build a common table that shows which users performed which categories of action on each day in each course
* Generate downstream aggregates from the common table
* Introduce a calendar table (which can later be extended to include complex things like fiscal year/quarter stuff)

If this approach looks good I'll write a bunch of unit tests for the calendar table, and increase the coverage of the user activity computation.

@brianhw @rocha
Yeah, it was a suggestion. `dateutil` is pretty useful, but not necessary at the moment. I disagree on the overhead, which I don't think it is very much operationally. On the other hand, you would have saved some time coding if you used it instead.