Add aria labels to account manager nav elements

The account manager prototype currently had two navigation landmarks,
one being in the site header, and another being the account menu.

WCAG SC 2.4.1 states that when there are multiple navigation landmarks
of the same kind on a page, they should be labelled in order to help
assistive tech users differentiate between them.

This adds relevant labels to these landmarks to improve navigation for
screen reader users.

app/views/application/_account-navigation.html.erb

@@ -1,6 +1,6 @@
 <% page_is ||= nil %>
 
-<nav>
+<nav aria-label="Account management">
   <ul class="accounts-menu">
     <li class="accounts-menu__item <%= "accounts-menu__item--current" if page_is == "your-account" %>">
       <a class="accounts-menu__link govuk-link govuk-link--no-visited-state" href="<%= user_root_path %>"><%= t("navigation.menu_bar.account.link_text") %></a>

app/views/layouts/application.html.erb

@@ -59,6 +59,7 @@
   <%= render "govuk_publishing_components/components/layout_header", {
     product_name: "Account",
     navigation_items: navigation_items,
+    navigation_items_label: "Account access",
   } %>
 
   <div class="govuk-width-container app-width-container">