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 was hoping to just use one of them, but I couldn't find anything related to ISO weeks in dateutil (granted I didn't look very hard). I would strongly prefer we stick to ISO weeks since it is very easy to describe and very clearly defined. Lots of different libraries deal with weeks differently so I wanted to stick to a standard.