Unitless line-height

* Instead of 20px as @line-height-base, use 1.5
* Update typographic scale for headings to use unitless as well--required some twerking of decimals a bit and some rounding
* Introduce new @line-height-computed value, which takes base font-size and multiplies it by 1.5, for use in padding and margin on components

docs/assets/css/docs.css

@@ -599,6 +599,7 @@ input.focused {
 }
 .highlight pre {
   padding: 0;
+  margin-top: 0;
   margin-bottom: 0;
   background-color: transparent;
   border: 0;