Add static spacing override classes to overrides

New spacing override classes which use the static spacing at all
viewports. The existing spacing override classes use the
responsive spacing helper, and so have different spacing at
different viewports. These helper classes are used in components in
govuk_publishing_components and the option to use static spacing
would be useful in specific areas of GOV.UK and as an option for
components.

src/govuk/overrides/_spacing.scss

@@ -2,6 +2,8 @@
 /// @group overrides
 ////
 
+// stylelint-disable declaration-no-important
+
 /// Directions for spacing
 ///
 /// @type Map
@@ -14,25 +16,27 @@ $_spacing-directions: (
   "left"
 ) !default;
 
-/// Spacing override classes
+/// Generate responsive spacing override classes
 ///
 /// Generate spacing override classes for the given property (e.g. margin)
-/// for each point in the spacing scale.
+/// for each point in the responsive spacing scale.
 ///
 /// @param {String} $property - Property to add spacing to (e.g. 'margin')
 ///
-/// @example scss
-///   .govuk-\!-margin-0 {
-///      margin: 0;
-///   }
+/// @example css
+///  .govuk-\!-margin-4 {
+///    margin: 15px !important;
+///  }
 ///
-///  .govuk-\!-margin-top-1 {
-///     margin-top: [whatever spacing point 1 is...]
-///   }
+///  @media (min-width: 40.0625em) {
+///    .govuk-\!-margin-4 {
+///      margin: 20px !important;
+///    }
+///  }
 ///
 /// @access private
 
-@mixin _govuk-generate-spacing-overrides($property) {
+@mixin _govuk-generate-responsive-spacing-overrides($property) {
   // For each point in the spacing scale (defined in settings), create an
   // override that affects all directions...
   @each $scale-point, $scale-map in $govuk-spacing-responsive-scale {
@@ -52,7 +56,38 @@ $_spacing-directions: (
   }
 }
 
+/// Generate static spacing override classes
+///
+/// Generate spacing override classes for the given property (e.g. margin)
+/// for each point in the non-responsive spacing scale.
+///
+/// @param {String} $property - Property to add spacing to (e.g. 'margin')
+///
+/// @example css
+///   .govuk-\!-margin-static-4 {
+///      margin: 20px !important;
+///   }
+///
+/// @access private
+@mixin _govuk-generate-static-spacing-overrides($property) {
+  @each $spacing-point in map-keys($govuk-spacing-points) {
+    .govuk-\!-static-#{$property}-#{$spacing-point} {
+      #{$property}: govuk-spacing($spacing-point) !important;
+    }
+
+    @each $direction in $_spacing-directions {
+
+      .govuk-\!-static-#{$property}-#{$direction}-#{$spacing-point} {
+        #{$property}-#{$direction}: govuk-spacing($spacing-point) !important;
+      }
+    }
+  }
+}
+
 @include govuk-exports("govuk/overrides/spacing") {
-  @include _govuk-generate-spacing-overrides("margin");
-  @include _govuk-generate-spacing-overrides("padding");
+  @include _govuk-generate-responsive-spacing-overrides("margin");
+  @include _govuk-generate-responsive-spacing-overrides("padding");
+
+  @include _govuk-generate-static-spacing-overrides("margin");
+  @include _govuk-generate-static-spacing-overrides("padding");
 }