Move admin index to a component

This way we make it easier to modify.

Note that, since the title of the page is "Administration" and it's in
the "Admin" section, we're adding an option to the `header` method in
order to avoid having a confusing title like "Administration - Admin".

Also note that, by removing the `title` HTML class, we avoid inheriting
styles from the `dashboard.scss` stylesheet, and now the heading is
displayed in the position it was meant to.

Finally, the concept of using a `main-class` for the current page comes
from a branch (currently in development) which will replace the <div>
tag with the `admin-content` class with a `main` tag.

app/views/layouts/admin.html.erb

@@ -21,7 +21,7 @@
         <% end %>
       </nav>
 
-      <div class="admin-content">
+      <div class="admin-content <%= yield(:main_class) %>">
         <%= label_tag :show_menu, t("admin.menu.admin"),
                       "aria-hidden": true, class: "button hollow expanded" %>