[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.
My understanding of SubFieldBase is that all it does is call `to_python` whenever any value is assigned to the field.

Personally, I wish it were on DateField and friends: being able to pass ISO8601 formatted strings and have them be date/time objects immediately makes for neater test cases.