wiki: improve how styles are loaded

 * Change loading of the main site's style.css to use the file
   provided by the tracker, because it is served via https.
   This prepares the move of the wiki to a new hoster, as style.css
   does not have to served from there. It has the drawback that
   the tracker site has to be up for the wiki style to work.

Author: bernhardreiter

website/wiki/static/roundup/css/screen.css

@@ -3,8 +3,10 @@
 Copyright (c) 2001, 2002, 2003 by Juergen Hermann
 */
 
-/* include style of main Roundup site */
-@import url(/_static/style.css);
+/* include style of main Roundup site.
+ * use the one from the tracker because it is available via https.
+ */
+@import url(https://issues.roundup-tracker.org/@@file/style.css);
 
 ul.editbar
 {