We had two places where datetimes were serialized to chop off the

extra precision. Turns out we needed a third, for json_dumps. These

other places also didn't know how to handle lazy strings, which only

json_dumps knew how to do.
There's now a DjbletsJSONEncoder that can take on these

responsibilities. BasicAPIEncoder, ResourceAPIEncoder, and json_dumps

now make use of this instead of DjangoJSONEncoder.

RB and Djblets unit tests pass.
Verified that the timestamps from json_dumps no longer had the

extra precision that was causing unwanted message bubbles in RB.