[contrib.postgres] UUIDField

Added UUIDField. Not particularly complicated this one - no custom lookups or anything. Casts all values to `uuid.UUID` instances in python.

TODO:
- [x] More form field tests
- [x] Documentation

Squashed patch should be marked as fixing #19463.
Yeah, I was wound up on the fact that my similar field was not setting the value correctly on a default, but I was doing something stupid. The `default=` is a better pattern (and is extensible to dates, etc).