CLI: Fix CSS path

CLI commands which require our LESS parser had no access to our CSS
because the public path was set to the path of the icingacli executable
which is most likely bin.

library/Icinga/Web/StyleSheet.php

@@ -83,7 +83,7 @@ class StyleSheet
         $app = Icinga::app();
         $this->app = $app;
         $this->lessCompiler = new LessCompiler();
-        $this->pubPath = $app->getBootstrapDirectory();
+        $this->pubPath = $app->getBaseDir('public');
         $this->collect();
     }