Notifies user when messages are not being marked as read through a
banner that lets them mark all messages in the narrow as read. Note
that the banner is only displayed if the user's actions, like
scrolling, would've actually marked the messages as read.
This avoids distracting the user when viewing a thread they've already
read.
tabbott has verified that if new messages come in, the banner will reappear.
Fixes: #18768.
Dinesh