Removed focus from slider after use

refs https://github.com/TryGhost/Team/issues/1249

If we leave focus on the slider than after interacting with it, it will
capture scroll events and move the slider rather than scrolling the
page.

core/frontend/src/cards/js/before-after.js

@@ -22,6 +22,10 @@
             updateSlider();
         });
 
+        input.addEventListener('change', function () {
+            input.blur();
+        });
+
         window.addEventListener('resize', function () {
             updateDimensions();
         });