Djblets gained a TokenExpiresFieldType, which was a

DateTimeFieldType subclass that allowed for empty strings.
We'll eventually want to allow fields like DateTimeFieldType to

optionally take an empty value, but we'll want to do that through an

official mechanism that all field types can opt into. Since this

particular class is very specific to APITokenResource, this change

moves it into that module, making it effectively a private

implementation detail.
The field type will be removed from Djblets, so we don't have to worry

about supporting it and then deprecating it down the road.

API token resource unit tests pass.