update all spacing variables in _custom-variables

[guylepage3]

Go through _custom-variables.scss and adjust to new spacing variables from custom spacing in previous blockstack-bootstrap``v4-alpha2

See #64

// 3. Spacing
//
// Control the default styling of most Bootstrap elements by modifying these
// variables. Mostly focused on spacing.
// You can add more entries to the $spacers map, should you need more variation.

$spacer:   1rem !default;
$spacer-x: $spacer !default;
$spacer-y: $spacer !default;
$spacers: (
  0: (
    x: 0,
    y: 0
  ),
  1: (
    x: ($spacer-x * .25),
    y: ($spacer-y * .25)
  ),
  2: (
    x: ($spacer-x * .5),
    y: ($spacer-y * .5)
  ),
  3: (
    x: $spacer-x,
    y: $spacer-y
  ),
  4: (
    x: ($spacer-x * 1.5),
    y: ($spacer-y * 1.5)
  ),
  5: (
    x: ($spacer-x * 3),
    y: ($spacer-y * 3)
  )
) !default;
$border-width: 1px !default;

Also to be defined is utilities/_spacing.scss

// Width and height

.w-100 { width: 100% !important; }
.h-100 { height: 100% !important; }

// Margin and Padding

.mx-auto {
  margin-right: auto !important;
  margin-left:  auto !important;
}

@each $breakpoint in map-keys($grid-breakpoints) {
  @each $prop, $abbrev in (margin: m, padding: p) {
    @each $size, $lengths in $spacers {
      $length-x:   map-get($lengths, x);
      $length-y:   map-get($lengths, y);

      @include media-breakpoint-up($breakpoint) {
        $min: breakpoint-min($breakpoint, $grid-breakpoints);

        @if $min {
          // everything else
          @media (min-width: $min) {
            .#{$abbrev}-#{$breakpoint}-#{$size} { #{$prop}:        $length-y $length-x !important; } // a = All sides
            .#{$abbrev}t-#{$breakpoint}-#{$size} { #{$prop}-top:    $length-y !important; }
            .#{$abbrev}r-#{$breakpoint}-#{$size} { #{$prop}-right:  $length-x !important; }
            .#{$abbrev}b-#{$breakpoint}-#{$size} { #{$prop}-bottom: $length-y !important; }
            .#{$abbrev}l-#{$breakpoint}-#{$size} { #{$prop}-left:   $length-x !important; }
            .#{$abbrev}x-#{$breakpoint}-#{$size} {
              #{$prop}-right:  $length-x !important;
              #{$prop}-left:   $length-x !important;
            }
            .#{$abbrev}y-#{$breakpoint}-#{$size} {
              #{$prop}-top:    $length-y !important;
              #{$prop}-bottom: $length-y !important;
            }
          }
        } @else {
          // xs
          .#{$abbrev}-#{$size} { #{$prop}:        $length-y $length-x !important; } // a = All sides
          .#{$abbrev}t-#{$size} { #{$prop}-top:    $length-y !important; }
          .#{$abbrev}r-#{$size} { #{$prop}-right:  $length-x !important; }
          .#{$abbrev}b-#{$size} { #{$prop}-bottom: $length-y !important; }
          .#{$abbrev}l-#{$size} { #{$prop}-left:   $length-x !important; }
          .#{$abbrev}x-#{$size} {
            #{$prop}-right:  $length-x !important;
            #{$prop}-left:   $length-x !important;
          }
          .#{$abbrev}y-#{$size} {
            #{$prop}-top:    $length-y !important;
            #{$prop}-bottom: $length-y !important;
          }
        }

      }
    }
  }
}

// Positioning

.pos-f-t {
  position: fixed;
  top: 0;
  right: 0;
  left: 0;
  z-index: $zindex-navbar-fixed;
}