Support downloading media

Add a function for downloading media, and use it as the default action for m.audio, m.file and m.video. Now that authenticated media is the expected situation opening these as links in an external browser no longer works, so try this instead. "Open in the chosen application for this mime type" would be nice as well, but based on looking at the way Gnus does this it seems harder than expected. Perhaps there is something I don't understand here..