Several fixes for styling of the "blank modal" selector

Several fixes for styling of the "blank modal" selector

This is the modal dialog that pops up when the user clicks on a
blank tile to select what letter it represents. The changes in
this commit are:

1. Use the same appearance (referencing the same CSS class) for
   the tiles in this modal as for tiles in other contexts, (this
   replaces the previous custom styling which was white text on
   a light background and hard to read).

2. Restructure the modal so that it is centered over the grid
   rather than centered over the viewport (also constrain the
   rack to not be wider than the grid).

3. Fix a bug that resulted from the user cancelling a selection of a
   blank tile, (it was "returned" to the rack before being properly
   removed from the rack, resulting in two occurences of the same tile
   on the rack). See the reassign_true logic in this commit for the
   relevant fix.