Use --end class for header nav

Uses `govuk-header__navigation--end` class which
does away with the need for a custom override.

Based on suggestion by @quis:

https://github.com/alphagov/notifications-admin/pull/3181#discussion_r361977771

app/assets/stylesheets/govuk-frontend/overrides.scss

@@ -1,16 +1,5 @@
 // Overrides for certain GOV.UK Frontend styles to make its components match this application's design
 
-// Notify's header navigation aligns to the right
-.govuk-header__navigation {
-  text-align: right;
-
-  // reset the alignment of each item to left-to-right
-  .govuk-header__navigation-item {
-    text-align: left;
-  }
-
-}
-
 // Additional padding-bottom override, following the GOV.UK Frontend spacing scale:
 // https://design-system.service.gov.uk/styles/spacing/#the-responsive-spacing-scale
 .govuk-\!-padding-bottom-12 {

app/templates/admin_template.html

@@ -124,7 +124,8 @@
   {{ govukHeader({
     "homepageUrl": url_for('main.show_accounts_or_dashboard'),
     "productName": "Notify",
-    "navigation": navigation
+    "navigation": navigation,
+    "navigationClasses": "govuk-header__navigation--end"
   }) }}
 {% endblock %}