Worksheet should be linked to Subject

[Matt Giuca]

Worksheet table in the DB has a 'subject' column, but it's just a string,
not a reference to a subject. This is used as part of the unique identifier
for a worksheet.

This should be modified to be a subject ID, not a subject name, and the ORM
side should change so that it's a Subject object, not name.

The final change is to replace store.find calls (which search on subject
name and worksheet name) with a method of Subject which looks up a
worksheet by name.