Move visited modifier so it does not conflict

By moving this selector before the original `.govuk-link` selector we can avoid a conflict which stops focused links from having a text colour.

src/core/_links.scss

@@ -1,5 +1,24 @@
 @include govuk-exports("govuk/core/links") {
 
+  // 'No visited state' link mixin
+  //
+  // Used in cases where it is not helpful to distinguish between visited and
+  // non-visited links.
+  //
+  // For example, navigation links to pages with dynamic content like admin
+  // dashboards. The content on the page is changing all the time, so the fact
+  // that you’ve visited it before is not important.
+  //
+  // This is not abstracted as a mixin because it does not provide states for
+  // all pseudo-selectors so it does not make sense to use it in composition.
+  //
+  // This is required to be before the root selector for specificity reasons.
+  .govuk-link--no-visited-state {
+    &:visited {
+      color: $govuk-link-colour;
+    }
+  }
+
   %govuk-link {
     @include govuk-link-common;
     @include govuk-link-style-default;
@@ -20,21 +39,4 @@
   .govuk-link--text-colour {
     @include govuk-link-style-text;
   }
-
-  // 'No visited state' link mixin
-  //
-  // Used in cases where it is not helpful to distinguish between visited and
-  // non-visited links.
-  //
-  // For example, navigation links to pages with dynamic content like admin
-  // dashboards. The content on the page is changing all the time, so the fact
-  // that you’ve visited it before is not important.
-  //
-  // This is not abstracted as a mixin because it does not provide states for
-  // all pseudo-selectors so it does not make sense to use it in composition.
-  .govuk-link--no-visited-state {
-    &:visited {
-      color: $govuk-link-colour;
-    }
-  }
 }