Rename white_square to white_large_square

Github doesn't use the ```:white_square:``` :white_square: [emoji](https://github.com/github/gemoji/tree/master/images/emoji).

Instead it has different sizes:
* ```:white_large_square: ``` :white_large_square: 
* ```:white_medium_small_square:``` :white_medium_small_square: 
* ```:white_small_square:``` :white_small_square: 

It seems that the :white_large_square: is the correct one to replace.