zulip / zulip-mobile / 3550

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webview: Better match the invoke-lightbox logic to the webapp.

When the user touches an image in the message list, we've been
deciding whether to invoke the lightbox for it based on whether its
parent element is a link with `target="_blank"`.  This is unsemantic
and kind of quirky, and in fact it doesn't always get the right
answer; for example, this was causing us to pull up a (failed,
blank) lightbox for the avatar in an embedded tweet, or for the
giant file-type icon in an embedded Dropbox link.

Instead, use the rather more semantically plausible test found in
the webapp.

Also add a few comments; explain in particular the "video" exceptions.

This code still isn't quite right, and the difference shows up in
the case of an embedded Dropbox *image*: we should be getting the
image URL from, well, the `img` element, but instead we're getting
it from the enclosing link, which has a different job and in the
case of a Dropbox image points to the `?dl=0` HTML page which
displays the image.  In that case, a lightbox is the right thing,
but because we use the wrong URL we show a blank one.  Leave that as
a TODO to be fixed separately, along with thumbnailing-awareness.

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