media assets: add thumbnail view to /media_assets page.

Add a thumbnail view to the /media_assets page. This page lets you see
all images uploaded to Danbooru by all users (although you can't see who
the uploader is). Also add a link to this page in the subnav bar on the
upload page.

app/components/media_asset_preview_component/media_asset_preview_component.html.erb

@@ -1,5 +1,5 @@
 <%= tag.article class: "media-asset-preview media-asset-preview-#{size}" do -%>
-  <%= link_to link_target, class: "inline-block relative", draggable: "false" do -%>
+  <%= link_to link_target, class: "inline-block relative flex justify-center", draggable: "false" do -%>
     <% if media_asset.is_animated? %>
       <div class="media-asset-animation-icon absolute top-0.5 left-0.5 p-0.5 m-0.5 leading-none rounded text-xs font-arial font-bold">
         <span class="media-asset-duration align-middle">
@@ -21,4 +21,8 @@
       <%= tag.img src: variant.file_url, width: variant.width, height: variant.height, class: "media-asset-preview-image w-auto h-auto max-h-#{size}px #{"max-w-full" if shrink_to_fit}", crossorigin: "anonymous", draggable: "false" -%>
     </picture>
   <% end %>
+
+  <div>
+    <%= footer %>
+  </div>
 <% end %>