New position utils, split docs

- Renames .pos-f-t to .fixed-top, adds .fixed-bottom and .sticky-top
- Updates utilities Sass to be split across more files (one for position, sizing, and spacing)

scss/utilities/_spacing.scss

@@ -1,14 +1,3 @@
-// Width and height
-
-@each $prop, $abbrev in (width: w, height: h) {
-  @each $size, $length in $sizes {
-    .#{$abbrev}-#{$size} { #{$prop}: $length !important; }
-  }
-}
-
-.mw-100 { max-width: 100% !important; }
-.mh-100 { max-height: 100% !important; }
-
 // Margin and Padding
 
 @each $breakpoint in map-keys($grid-breakpoints) {
@@ -52,13 +41,3 @@
     }
   }
 }
-
-// Positioning
-
-.pos-f-t {
-  position: fixed;
-  top: 0;
-  right: 0;
-  left: 0;
-  z-index: $zindex-navbar-fixed;
-}