Instead of having to view a LINE chat when a media message is sent, send
a placeholder message that gets replaced with the actual media when it's
viewed in Matrix.
Use the sidebar to sync DM messages instead of visiting the DM itself,
so as to not make LINE (and your contact) think you really read the DM.
This cannot be done for non-text messages (which are not previwable in
the sidebar) and non-DM chats (whose sidebar messages don't say who sent
a message).
