Move TokenExpiresFieldType from Djblets into Review Board.

Review Request #12614 — Created Sept. 20, 2022 and submitted — Latest diff uploaded

Review Board

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.