Give outline reset same precedence as for focus

The CSS that cancelled outline on focus events not
fired by the :focus-visible heuristic is being
overridden by the higher precedence of the outline
style for :focus, due to its use of !important.

This adds !important to the cancelling CSS. This
brings that block up to the same level as that for
:focus, meaning the :focus-visible styles will win
because they sit lower in the stylesheet.

app/assets/stylesheets/map.scss

@@ -32,7 +32,7 @@ $zoom-button-hover-colour: govuk-shade($zoom-button-colour, 10%);
   // from other sources (like clicks on child elements).
   &:focus:not(:focus-visible) {
     box-shadow: none;
-    outline: none;
+    outline: none !important; // sass-lint:disable-line no-important
   }
 }