Added styles.php and scripts.php

These are included in every site and include generally needed stylesheets and scripts such as jquery and bootstrap.
Depending on the configuration, these scripts decide whether to include the stylesheets and scripts as local file copies or from an official CDN network.
The first option should be used for local networks without access to the internet, the second option should be used for open networks with access to the internet because it provides better caching behaviour.