[REF] web: introduce a way to extend the bg-variant mixin

Needed for themes migration.

addons/web/static/src/scss/bs_mixins_overrides.scss

@@ -14,6 +14,8 @@
                 color: rgba($-yiq-color, 0.4) !important;
             }
         }
+
+        @include o-bg-color-extension($color, $text-color, $with-muted);
     }
 }
 

addons/web/static/src/scss/utils.scss

@@ -154,6 +154,10 @@
     }
 }
 
+
+// Mixin which allows to extend the BS4 bg-variant mixin
+@mixin o-bg-color-extension($color, $text-color, $with-muted) {}
+
 // Function to remove all falsy values of a map
 @function o-map-omit($map) {
     $-map: ();