Fixes Insufficient colour contrast on select boxes on Windows 10

In Windows 10 on Internet Explorer and Edge selected and active
options in select boxes have a light blue visual treatment.
That differes from other browser vendors' implementation and it
it has a contrast ration 2.9:1 which is not accesible.
By applying microsoft only CSS values  using the GOV.UK
blue this has now been fixed.

public/sass/elements/_forms.scss

@@ -9,6 +9,7 @@
 // 4. Form hints
 // 5. Form controls
 // 6. Form control widths
+// 7. Browser accessibility fixes
 
 // 1. Helpers
 // ==========================================================================
@@ -223,3 +224,13 @@ textarea.form-control {
     width: 12.5%;
   }
 }
+
+// 7. Browser accessibility fixes
+// ==========================================================================
+
+option:active,
+option:checked,
+select:focus::-ms-value {
+  color: $white;
+  background-color: $govuk-blue;
+}