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.
No, the `max_length` argument is ignored (or forced) to be 32.