Graceful error-handling with imports and defining where import files are stored

- Import files from different providers are added to define.inc.php as configuration values
- When calling the import action ($_GET['import'] is set), there is a check whether a routine for that provider actually exists
- When the routine exists, there is a check whether the required import file location is set in define.inc.php
- There is a check whether the targeted import file actually exists (partially fixes #172)
- config.twig its import section reflects the required files defined in define.inc.php