While adding the UI for setting expiration dates on Review Board's API tokens,

we ran into a blocker for setting the expiration to a null value (i.e. giving

the token no expiration date). This is because our API request fields don't

support null values, they expect a value to be set for the given field.
Rather than tackling this concept of allowing null values for all request

fields right now (this should be done eventually though), this change simply

creates a specialized TokenExpiresFieldType which will allow null values as

well as date/time values that are supported by the DateTimeFieldType.

Created unit tests for the new field type and ran tests in

djblets.webapi.tests.test_fields.py
Manually tested setting the expiration date to empty strings and

  null values through the API.