Add subscribe/unsubscribe marker lines to message list (historical flag)

[neiljp]

This issue is mainly to track this feature and needs more investigation before proceeding.

However, from semi-recent discussion, this may be as simple as when the historical flag flips between adjacent messages.
eg. a bug reported at https://chat.zulip.org/#narrow/stream/2-general/topic/.28Un.29subscribe.20UI.20event.20data (#general > (Un)subscribe UI event data)

Note that currently one can only access subscribed streams, but this flag should be on messages before subscribing if one scrolls back (up). Testing that would be a good starting point.

This may be a good first issue, depending if you wish to do a deep dive into a lot of the structure of message data and rendering of the message list.