[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