adjust models so they are fit for a web app

[fgregg]

The minimum that this would require is adding a nullable slug field to

• Event
• Bill
• Person
• Organization

I am not advocating that pupa set these slugs, just that they exist on the models and could be filled in later.

Very useful:

• turn the image attribute on person from a UrlField to a FileField (headshots)
• turn all the various dates and datetimes from strings to native dates and datetimes.

To effectuate these changes, pupa would have to be adjusted.

This should drastically reduce the need for the councilmatic_core one-to-one models and would make queries much simpler throughout the web apps.