Added the configuration variable $conf['demo_mode'] in the config.inc.php file. It disables the newly introduced demo mode which limits certain actions in the interface part of ispconfig: the demo mode can be used to run the ispconfig interface part as online demo.

Fixed: FS#846 - Language Editor: wrong filename shown while editing language-file outher global module 

- Changed shell of website linux user to /bin/false

* introduciton ISPC_WEB_PATH and ISPC_THEMES_PATH