This endpoint serves requests which might originate from an image
preview link which had an http url and the message holding the image
link was rendered before we introduced thumbnailing. In that case
we would have used a camo proxy to proxy http content over https and
avoid mix content warnings.
In near future, we plan to drop use of camo and just rely on thumbor
to serve such images. This endpoint helps maintain backward
compatibility for links which were already rendered.
This will help us eliminate camo from our production installs.
Basically it helps us de duplicate some code from upcoming code
which will help us check validity of a camo url.
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]]`.
