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.