Breakpoints / Media Queries refactor

- Rename `$a-leftCol-width` into `$left-column`
- Rename `$a-leftColWide-width` into `$left-column-wide`
- Remove unused breakpoint `desktopAd`
- Remove unused variable `$unitHeight`
- Replace `gs-span(2)` and `gs-span(3)` in code by `$left-column` and `$left-column-wide` where applicable

I kept breakpoint names the same to avoid disturbing other devs. Plus, existing names have served us well so far.

/cc @phamann this should solve the issue in `_facia-container.scss` where we had a rogue breakpoint

![ ](http://media.giphy.com/media/rk7ds6R0RMpr2/giphy.gif)
See #5467 as the promised run-through.