Fix old Gitter links #3342
Description
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 matrix-org/matrix-viewer#288 (comment) (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?)