Simplify padding rules in SDG pages

We can give the padding to the whole page instead of giving it to
individual elements.

On the minus side, now padding in the SDG pages is not the same as the
padding in the homepage, so we need to add an extra padding to the
participation feeds in only one of these cases.

app/assets/stylesheets/layout.scss

@@ -2617,6 +2617,10 @@ table {
   }
 }
 
+.home-page .feeds-participation {
+  @include grid-column-gutter;
+}
+
 .feeds-list {
 
   a {

app/assets/stylesheets/sdg/goals/index.scss

@@ -1,10 +1,9 @@
 .sdg-goals-index {
+  @include grid-column-gutter;
 
   .section-header {
 
     h1 {
-      @include grid-column-gutter;
-
       &::before {
         @extend %font-icon;
 
@@ -21,7 +20,6 @@
     $spacing: 1.5%;
 
     @include sdg-goal-list($spacing);
-    @include grid-column-gutter;
 
     margin-left: auto;
     margin-right: auto;
@@ -41,7 +39,6 @@
   }
 
   .sdg-phase {
-    @include grid-column-gutter;
 
     .cards-container {
       @include grid-row-nest;

app/assets/stylesheets/sdg/goals/show.scss

@@ -1,8 +1,5 @@
 .sdg-goal-show {
-
-  > :not(.feeds-participation):not(.sdg-goal) {
-    @include grid-column-gutter;
-  }
+  @include grid-column-gutter;
 
   .sdg-goal {
 
@@ -29,7 +26,6 @@
     }
 
     h1 {
-      @include grid-column-gutter;
       align-items: center;
       display: flex;
       font-family: "Oswald" !important;
@@ -55,7 +51,7 @@
 
     @each $code, $color in $sdg-colors {
       &.sdg-goal-#{$code} > header {
-        @include full-width-background;
+        @include full-width-background($adjust-padding: true);
         background-color: $color;
 
         &::after {
@@ -65,7 +61,6 @@
     }
 
     .goal-description-content {
-      @include grid-column-gutter;
 
       .read-more {
         text-align: center;

app/assets/stylesheets/widgets/feeds/participation.scss

@@ -1,6 +1,5 @@
 .feeds-participation {
   $gap: rem-calc(map-get($grid-column-gutter, medium));
-  @include grid-column-gutter;
   display: flex;
   flex-wrap: wrap;
   margin-left: -$gap;