Turn outlines off on map control buttons

Leaflet does this anyway when they're focused
(through JS) but we found holding shift when on a
focused button, which you do when tabbing
backwards, turns this off for some reason so you
see the outline the browser applies by default.

This turns all outlines off to stop that
happening.

Worth nothing that focus is indicated by:
- a change of background colour instead when
  tabbing
- a border in forced-color mode

...so the outline is not needed.

app/assets/stylesheets/map.scss

@@ -61,6 +61,9 @@ $zoom-button-hover-colour: govuk-shade($zoom-button-colour, 10%);
 
   a {
 
+    // Outlines don't work because of the buttons being so close together so turn off
+    outline: none;
+
     &:last-child {
       // Allow it to contain the absolutely positioned divider bar we show between buttons
       // when one is focused