Replaced a {{ ADMIN_MEDIA_URL }} reference by a {% admin_media_prefix…

… %} to avoid image 404.

This was causing issues when viewing a page version since the images were loaded relatively to
the page since ADMIN_MEDIA_URL was an empty string.

cms/templates/admin/cms/page/change_form.html

@@ -36,7 +36,7 @@
 	margin-bottom:-1px;
 	border-color:#ddd #aaa #ccc #ddd !important;
 	border:1px;
-	background:#fff url('{{ ADMIN_MEDIA_URL }}img/admin/nav-bg.gif') repeat-x center bottom !important;
+	background:#fff url('{% admin_media_prefix %}img/admin/nav-bg.gif') repeat-x center bottom !important;
 }
 
 #lang_tab_content h2.header {
@@ -45,7 +45,7 @@
 	font-size:11px;
 	text-align:left;
 	font-weight:bold;
-	background:#7ca0c7 url('{{ ADMIN_MEDIA_URL }}img/admin/default-bg.gif') repeat-x left top;
+	background:#7ca0c7 url('{% admin_media_prefix %}img/admin/default-bg.gif') repeat-x left top;
 	color:#fff;
 }