The WebAPIToken code used to live in Review Board, and was moved to

Djblets. When that happened, the local_site argument to the function was

kept, unintentionally. We still need it, though, and callers will need

to be able to set their own custom token model fields when generating a

token, so generate_token() now accepts keyword arguments, passing them

along to the model.
Calling this function could still be a bit confusing, though, as some

arguments seemed optional but would trigger strange errors when saving

the token. These were due to the fields not working with None values. We

now set sane types and normalize values when saving, helping with the

default case of not passing all arguments.

Djblets and Review Board unit tests pass.
Tested manually with a new unit test in Review Board I had worked on,

which hit the issues with the default values for optional parameters.