AO3-6553 Turn homepage media list into navigation

Semantically, the list of media (plus All Fandoms) is a navigation
element, not a menu.

The preferred element to use would be `nav`, but we don't use this
anywhere else (yet), so for consistency I'm adding a `div` with the role
navigation.

(Per https://www.w3.org/TR/html-aria/#docconformance, `ul`s are not
allowed to have the role `navigation`.)

I've also done the two "easy" translation strings.

app/views/menu/_menu_fandoms.html.erb

@@ -1,10 +1,12 @@
-<ul class="menu" role="menu">
-  <li><%= link_to ts('All Fandoms', key: 'header.fandom'), media_path %></li>
-  <% cache "menu-fandoms-version4", skip_digest: true do %>
-    <% Media.for_menu.each do |medium| %>
-      <% unless medium.id.nil? %>
-        <li id="medium_<%= medium.id %>"><%= link_to ts("#{medium.name}", key: 'header.fandom'), medium_fandoms_path(medium) %></li>
+<div role="navigation" aria-label="<%= t(".media") %>">
+  <ul class="menu">
+    <li><%= link_to t(".all_fandoms"), media_path %></li>
+    <% cache "menu-fandoms-version4", skip_digest: true do %>
+      <% Media.for_menu.each do |medium| %>
+        <% unless medium.id.nil? %>
+          <li id="medium_<%= medium.id %>"><%= link_to ts("#{medium.name}", key: 'header.fandom'), medium_fandoms_path(medium) %></li>
+        <% end %>
       <% end %>
     <% end %>
-  <% end %>
-</ul>
+  </ul>
+</div>

config/locales/views/en.yml

@@ -450,6 +450,10 @@ en:
       faux_heading: 'Important message:'
       point1: You are using a proxy site that is not part of the Archive of Our Own.
       point2: The entity that set up the proxy site can see what you submit, including your IP address. If you log in through the proxy site, it can see your password.
+  menu:
+    menu_fandoms:
+      all_fandoms: All Fandoms
+      media: Media
   muted:
     mute: Mute
     muted_items_notice_html: You have muted some users on the Archive. Some items may not be shown, and any counts may be inaccurate. You can mute or unmute users on %{muted_users_link}.