Add UI + server message to mark messages unread

[neiljp]

Implement #1261 first, which is much higher priority to ensure data integrity is maintained while a terminal session is open.

This concerns adding primarily UI code to support the user marking messages as unread.

The mobile equivalent is zulip/zulip-mobile#5364, which has connected requirements.
(the 'not being marked read' is essentially a narrow-specific ZT 'explore mode', equivalent to the 'near' URLs)

Note that the UI may be partially blocked on zulip/zulip#22893, which the web app carefully bypasses and which mobile may also do.