[WIP] Toggle showing only unread streams

[neiljp]

This is a draft approach to filter the visible streams, leading to a result a little like the unreads view in the mobile app.

While this is a WIP, this is pretty useful already; next I'll likely look at filtering topics in a similar way.

Some outstanding points to consider:

• Position is saved and restored when toggling, but we don't take into account eg. no unread streams (or no subscribed!)
• Should the unread 'filter' state be different for topics and streams, and maybe even between each set of topics (ie. stream) - possibly easier for us and users to just have one setting?
• I left search looking over all streams (even if previously in unreads-only view), but this can leave a strange situation where deleting the search text gives a different view from the unread stream list, so I'm reconsidering this
• Muted streams are excluded from the unreads - this seems reasonable?
• Unlike the mobile app (?), "emptied" unread streams (ie. initially with unreads, but then up to date) are not updated, except with a UU ie. toggle to all and back to only unreads to re-filter
• Unread counts mostly just work - but I just noticed an exception that if messages arrive in currently not-shown (read) streams then they are not counted.
• The setting is not saved between sessions right now

[zulipbot]

Heads up @neiljp, we just merged some commits that conflict with the changes your made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/main branch and resolve your pull request's merge conflicts accordingly.