Create a variable for label margin bottom (#25561)

scss/_reboot.scss

@@ -309,7 +309,7 @@ th {
 label {
   // Allow labels to use `margin` for spacing.
   display: inline-block;
-  margin-bottom: .5rem;
+  margin-bottom: $label-margin-bottom;
 }
 
 // Remove the default `border-radius` that macOS Chrome adds.

scss/_variables.scss

@@ -383,6 +383,8 @@ $btn-transition:              color .15s ease-in-out, background-color .15s ease
 
 // Forms
 
+$label-margin-bottom:                   .5rem !default;
+
 $input-padding-y:                       $input-btn-padding-y !default;
 $input-padding-x:                       $input-btn-padding-x !default;
 $input-line-height:                     $input-btn-line-height !default;