Basic placeholder webui

Adds the bare minimum files to have a functional web interface with
placeholder sidebar, home and admin pages.

source/webui/templates/errors/404.tpl

@@ -0,0 +1,6 @@
+<h2>The requested page could not be found</h2>
+<p>
+	The file you requested ({$requested_page}) could not be found.
+	If you typed in the address manually, check that you have spelled it correctly,
+	or if you followed a link, let us know and we'll look into it. 
+</p>