Parse topic permalinks

[gnprice]

When parsing an internal link, we should handle the upcoming "topic permalink" feature:

• Provide permalinks for topics zulip#21505

In the current draft, these look like /with/NNNN, with a message ID.

Currently the app doesn't interpret /near/ links at all (that's #82 and #683). So we should handle these new /with/ links the same way for now — accept them and ignore them.

In the future when we implement #683, these new links should be included in that behavior, but unlike /near/ links they won't have the behavior of #82. (They'll have the behavior #80 instead, like other narrow links.)

[gnprice]

(Assigned and unassigned because I was looking at the wrong column in the project board; what I meant to do was to set the "owner", to place it in Sayed's section of the board. Did that now, and @sm-sayedi please go ahead and self-assign the issue whenever you start working on it.)