This'll be shown only when in a different narrow from what
you're composing to.
Takes care of updating display of the button on moving from
one narrow to another and also on changing inputs. This is
what contributes to majority of js code in this commit.
We are not displaying this for private messages since we do not
have a consistent design for both stream and private compose areas.
See https://chat.zulip.org/#narrow/stream/101-design/topic/narrow.20to.20topic.2Fpms.20when.20composing/near/1318548
Thanks to Vlad Korobov for the icon and for proposing various
designs.
Dinesh