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
I don't think it's worth the overhead of yet another third party package. If/when we need to do other complex date deltas that aren't handled by datetime.timedelta then we should definitely consider it.