Disconnect $base-spacing from $base-line-height

The two serve different purposes and their values don't need to be
relational. Also, since we do not facilitate or support a vertical
rhythm, the two variables do not need to be connected.

One situation where I've found this to be problematic (when they are
connected) is when I want to tweak the line height for a specific
typeface, but I don't want that change to manipulate spacing throughout
the whole application.

core/_variables.scss

@@ -8,7 +8,7 @@ $heading-line-height: 1.2;
 
 // Other Sizes
 $base-border-radius: 3px;
-$base-spacing: $base-line-height * 1em;
+$base-spacing: 1.5em;
 $small-spacing: $base-spacing / 2;
 $base-z-index: 0;