Implement gitter threads - import as some Solid equivalent

[timbl]

Make it s on the thread itself also has its own URI.
But keep it so you can read the chat with non-thread-aware solid clients.

Ideally upgrade all solid-clients to include threads!

[TallTed]

Consider NNTP as a better model than most (if not all) reinventions of threading over the past several years, and the best NNTP reader you've ever used as a model for use within Solid -- really, applicable to any and all messaging, including Gitter and other IM-like systems, GitHub Issues, GitHub Discussions, email, etc.