posts: add "Absurd" size option to thumbnail size menu.

Add option for "Absurd" (720px) size thumbnails to the thumbnail size
menu. 720px size thumbnails require .webp support so they won't work in
older browsers.

app/components/preview_size_menu_component/preview_size_menu_component.html.erb

@@ -29,4 +29,9 @@
   <% menu.item do %>
     <%= link_to "Gigantic", current_page_path(size: 360), class: ["desktop-only", ("font-bold" if current_size == 360)], rel: "nofollow" %>
   <% end %>
+
+  <%# Desktop only %>
+  <% menu.item do %>
+    <%= link_to "Absurd", current_page_path(size: 720), class: ["desktop-only", ("font-bold" if current_size == 720)], rel: "nofollow" %>
+  <% end %>
 <% end %>