Partially broken: get system list by vendor name.

This works if you pass in /system/vendor/HP (case sensitive) but the
idea of creating a link in the overall system list fails.  In fact,
just accessing /system causes an IndexError.  I don't fully understand
dynamic filtering in class-based views; this might be a 1.5 concept.
Disjointed (but hopefully correct) merge of new models from upstream
Make dates, notes optional for computers and pci devices
Model refinements based on admin interface testing.