This moves the logic for renaming a stream to the REST API
update_stream_backend method, eliminating the legacy API endpoint for
doing so.
It also adds a nice test suite covering international stream names.
This was the original way to send messages via the Zulip API in the
very early days of Zulip, but was replaced by the REST API back in
2013.
Fixes: #730.
For a long time, rest_dispatch has had this hack where we have to
create a copy of it in each views file using it, in order to directly
access the globals list in that file. This removes that hack, instead
making rest_dispatch just use Django's import_string to access the
target method to use.
[tweaked and reorganized from acrefoot's original branch in various
ways by tabbott]
