Force exclamation icon to stay the same size

With longer text in an `AppStatusComponent`, the blue exclamation mark icon was getting smaller. This forces it to stay the same size, and not shrink

Author: alistair-white-horne-tng

app/assets/stylesheets/components/_status.scss

@@ -8,8 +8,11 @@
   gap: nhsuk-spacing(1);
 
   .nhsuk-icon {
+    flex-shrink: 0;
+    height: 1.5em;
     margin-left: nhsuk-spacing(-1);
     margin-top: nhsuk-spacing(-1);
+    width: 1.5em;
   }
 
   &--aqua-green {