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
We can also use `dateutil`, it has a handly `relativedelta` object that makes month based operations very easy (it takes into account the differences in the number of days, etc).