Make small changes and additions to variables

* Rearrange size variables
* Add z-index variables
* Make header-line-height unitless
On the same note as my previous comment, what do you see as the use-case for `$base-vertical-spacing` vs. `$base-line-height`? Should `$base-line-height` become the unit-less value (then making `$unitless-line-height` go away) and this `$base-vertical-spacing` become a unit-based value for a baseline vertical alignment value?