Move message previews into RoomTile's state
Move message previews into RoomTile's state

Now that it doesn't re-render without state updates, we should just wedge it into state.
matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk