add due date attribute to activities

> 1) add due date attribute to activities
> this is required
> filter activities by due date
> 'term (drop-down) section (drop-down) (checkbox) show due in
> past n (1-9 drop-down) weeks'

We looked at how other web gradebooks handle fitting the large number
of columns/assignments one will accumulate over the course of a term
(semester). Right now, in SchoolTool's gradebook you can make
multiple worksheets (like a spreadsheet) and that gives you one way to
break it up.

The other method which seemed to give the most bang for the buck is
allowing filtering by due date, specifically a checkbox that says
"show activities due in the last (select #) weeks." So you can
usually only look at what was due in the last week or two, or uncheck
it to grade older assignments.

Of course, to do this you need a due date attribute, which we
obviously needed sooner or later. Post 1.0 we'll make the due dates
show up on student and teacher calendars.