The new method borrows some code from the event loop
and unread_ops.mark_messages_as_read, and it is now
flexible about message_ids being marked as unread
even when there is no corresponding message in the
message store. For that scenario we still want to
update our data structures, which wasn't happening
before this change. (Generally, this was a non-issue
up until now, but it will become a bigger issue when
we start loading unread message ids from the server.)
This function allows us to see whether unread.js thinks a message
id is unread (as opposed to looking at the message itself). This
method is useful when we get notifications from the server that a
message has been read. In the future, we may not actually have
a local copy of an unread message, but we'll still know that it is
unread based on page_params. We'll want to update the data in that
case.
Going forward, we'll want to deprecate message.flags for most use
cases and just use the unread.js data structures to track unread
messages.
