Highlight target message on following a /near/ link

[gnprice]

After #82 (e.g. through #1517), if the user follows a message link (one with /near/), the message feed will open with the target message in view, regardless of where it's found in the conversation's history.

It isn't necessarily easy to spot the specific message, though. It'd be useful to somehow make visibly clear which message was the target of the link. This is especially valuable when the person who posted the message link intended to specifically refer to that message, and expected their reader to see which message they meant because the web app does make that clear.

Design

The web app accomplishes this with the blue box / the pointer. That doesn't exist in the mobile app, because its primary value is in the context of keyboard shortcuts.

One design that could make sense is to have the target message's background blink once or twice, over a duration of perhaps a second or so. I've seen behavior something like that in Discord, I think, and it seems to work well there.