Use bill subject bulk data

[waldoj]

Every bill is assigned to a subject, as part of a two-level hierarchy. Those are stored in CIBillSubjects.csv and CIParentChildSubjects.csv, on the FTP site. Create a new table to store the subject hierarchy, and start storing the subject for every bill.

Eventually, this can be a new way to browse bills. For now, it's great hinting for tags.