Follow /near/ links through message moves

[gnprice]

When a Zulip narrow link has a /near/ operator in it, this means two things:

(A) Start the view around the given message, rather than at the user's first unread.

(B) If the message has moved — for example if the topic was renamed or resolved or split up — then find where the message is now, and go to that conversation instead of the one specified by any channel/stream or topic operators in the link.

Part A is #82. This issue is for part B.

The corresponding zulip-mobile issue, back in 2022 (when /near/ links gained that meaning in the first place), was zulip/zulip-mobile#5306 .

This issue is independent of #82, and we can do them in either order. (Web had part A without B for many years; zulip-mobile has part B but still lacks part A.)

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(Previously, this issue also tracked following /with/ links ("topic permalinks") through message moves. That's been split out to #1028, but at least one commit in main, ebb41b6, points to this issue as tracking the /with/ feature.)