Fix old Gitter links

It's annoying to read through code and then come across a Gitter link that does not work. While I can probably sometimes guess the message contents + search for it, this doesn't seem very fun or a good use of time. And generally I'm looking at the message for information I don't already know!!

I think we should set up some sort of archive of the Gitter channel for permanent links + update code links. Unfortunately, Matrix will probably never provide this service -- when they have in the past they faced immense community pushback -- so I think we'll need to step up and do it ourselves. See https://github.com/matrix-org/matrix-viewer/issues/288#issuecomment-3156830483 (and https://github.com/matrix-org/matrix-viewer more widely) for an existing solution, though I'm not sure it's easy to set up.

(Generally it's concerning that Gitter now requires login to view and if I were choosing a messaging platform nowadays I'd probably have gone with Zulip... But it's not worth the switching cost, I think?)

No files or tests are named in the issue. Start by locating the existing Gitter links in the repository and read the linked matrix-viewer issue and comment for the proposed archive approach. Done means a permanent archive exists and the affected code links have been updated, but the implementation scope is not yet defined.