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
Minor: I feel a bit of deja vu. Maybe the return type of `get_event_and_date_string` when there is no valid event could be `(None, None)`. That way we don't have to do the unpacking below.

It is not a big deal, just a little nicety.