Implement /user/@userid:domain?action=chat

This is a URL that can be used to start a chat with a user.
 - If the user is a guest, setMxId dialog will appear before anything and a defered action will cause `ChatCreateOrReuseDialog` to appear once they've logged in.
 - If the user is registered, they will not see the setMxId dialog.

fixes https://github.com/vector-im/riot-web/issues/4034

src/createRoom.js

@@ -97,6 +97,7 @@ function createRoom(opts) {
         // the room exists, causing things like
         // https://github.com/vector-im/vector-web/issues/1813
         if (opts.andView) {
+            console.info('And viewing');
             dis.dispatch({
                 action: 'view_room',
                 room_id: roomId,