Simplify the SCSS used for expanding the grid

I noticed that you hardcoded certain formulas for the pixel size of the grid, even though you added the cell size of the grid as a variable.  Really, the only stuff that's relevant is the stuff in [this commit](https://github.com/austinoneil/2048/commit/a5520294c3302a09fbb8acc258f4180fe71a96cc) on the main.scss file.