We used to create endpoints with Client._register.
Now we now have explicit methods for the endpoints.
This allows us to add docstrings and stricter mypy annotations.
This fix also introduces a call_endpoint() method that avoids
the need for manually building urls with API_VERSTRING when you
know the URL pattern of the endpoint you want to hit (and when
the API doesn't have a convenient wrapper).
I fixed a bug with create_users where it now uses PUT instead
of POST.
I also removed client.export(), which was just broken.
I had to change recent-messages and zulip-export, which were
using client.do_api_query and Client._register.
Now it's easier to just call client.call_endpoint() for
situations where our API doesn't have convenient wrappers,
so that's what I did with those scripts.
