Bookmarkable threads and perhaps messages

[struanb]

Update the URL dynamically as the user browses around threads (and perhaps messages) to allow these to be bookmarked (or cited in separate private documents, notes etc) for ease of reference.

This will only work with the web app, unless perhaps Prompta is converted to a full app at some point, but as long as a user is only using web, or has both PWA and web synced with the same db key, this would be really useful.

(Being able to share threads could be useful too, but that seems a separate issue).