admin: change path behaviour for the billionth time

URLs are made with app.ExternalURI (now included in window.pages), which
itself should include Base. Also fixed the subfolder being removed from
the url after login.

html/header.html

@@ -15,6 +15,8 @@
 <script>
     window.pages = {
         "Base": "{{ .pages.Base }}",
+        "TrueBase": "{{ .pages.TrueBase }}",
+        "ExternalURI": "{{ .pages.ExternalURI }}",
         "Current": "{{ .pages.Current }}",
         "Admin": "{{ .pages.Admin }}",
         "MyAccount": "{{ .pages.MyAccount }}",