Fixed #19463 -- Add 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.
You could make the same argument for all kinds of modules throughout django - starting with the import of `decimal` two lines above.

Python's `uuid` is a single python file. The overhead to import it at server-start time is so negligible it's not worth doing anything.