changed from include to require_once and include_once

In all the files ```include``` is used to include files, this may lead to one file being included twice. For example, both ```accounts/include/db.php``` and ```accounts/include/header.php``` include ```config.php``` . Now these  two files are included together in ```accounts/login.php``` thus including ```config.php``` twice. This is just one of the many cases. To prevent future problems (may be want config.php to generate validation errors in future, may be we don't want the scripts in header.php to be loaded again and again) 

*```require_once()``` for files that are required, without which the rest of things would stop working.
*```include_once()``` for files that must be included, but if not included rest of things may ignore and keep working.

I din't found a use case for ```include``` or ```require``` in the codebase.