use rtl version of css on admin

src/views/admin/header.tpl

@@ -5,7 +5,7 @@
 
 		{{{each metaTags}}}{function.buildMetaTag}{{{end}}}
 		{{{each linkTags}}}{function.buildLinkTag}{{{end}}}
-		<link rel="stylesheet" type="text/css" href="{relative_path}/assets/admin.css?{cache-buster}" />
+		<link rel="stylesheet" type="text/css" href="{relative_path}/assets/admin{{{ if (languageDirection=="rtl") }}}-rtl{{{ end }}}.css?{cache-buster}" />
 
 		<script>
 			var config = JSON.parse('{{configJSON}}');