WebAPIResource stores a copy of the generated serialized object for
later use. This is because the caller may modify the contents, breaking
what's later returned. We were using deepcopy() for this task, but that
behaved very badly with Django models.

We now have our own more light-weight function for safely cloning the
data we care about from a serialized object. Fewer things need to be
copied, and we know we're only duplicating what's needed.

Review Board and Djblets unit tests pass.
I didn't hit the breakage with Power Pack that I was previously hitting.