Fix people section again

- Alter CSS due to removed mx_RoomDropTarget_avatar. The avatar was removed because it didn't particularly add anything and we needed space for "Drop here to tag as direct chat", which is quite long.
 - Use guessAndSetDMRoom as a convenience method for guessing the DM member and setting the state.
 - Do evil hacks to make DNDRoomTile do dragging of RoomTiles to and from the People section. Dragging a DM to and from Rooms/Favourites/Low Priority now works as one would expect. This is still not ideal however because edge cases exist where you have more than one tag set and then you drag a DM from "Favourites" to "Rooms" and the tile ends up in "People". This would require setting multiple tags, and breaks the 1-1 mapping between tags and sections even further. Ultimately the UI needs a rework.

src/components/views/rooms/RoomDropTarget.js

@@ -31,7 +31,6 @@ module.exports = React.createClass({
         else {
             return (
                 <div className="mx_RoomDropTarget">
-                    <div className="mx_RoomDropTarget_avatar"></div>
                     <div className="mx_RoomDropTarget_label">
                         { this.props.label }
                     </div>