Also, fixed up the annotations for tornadoviews to better align with
how narrows was defined as `Iterable[Sequence[str]]` rather than
`List[Tuple[str, str]]`.
This will allow us to substantially decrease the server-side work that
we do to support our Mirroring systems (since the personal mirrors can
request only messages that user sent) and also is what we need to
support a single-stream Zulip widget that we embed in webpages.
(imported from commit 055f2e9a523920719815181f8fdb44d3384e4a34)