Reduce spacing between items in navigation bar

As per #0591877d96 in Mavis:

This is necessary because in the new schools and sessions design we have
added a new "Schools" menu item, which can sometimes cause the menu to
overflow.

Jira-Issue: MAV-2645

Author: mikejamesthompson

mavis/reporting/assets/scss/_header.scss

@@ -1,5 +1,9 @@
 @use "config" as *;
 
+.nhsuk-header__navigation-item {
+  padding-right: nhsuk-spacing(2);
+}
+
 .app-header__navigation-item--with-count {
   .app-count {
     margin-left: nhsuk-spacing(1);