new option apps_only to skip scan_repo_files

Currently, all files located in the repo/ sub directory (except for the index files) are considered to be "apps", and added to the index in scan_repo_files (fdroidserver/update.py). So if one for example places some index.php or index.html there to prevent the web server from showing an index of all files (which may be needed on hosted solutions where one has no access to the server configuration), those will end up in the index. At the moment that even leads to fdroidclient crashing when the user wants to view this "app".

This MR introduces an option apps_only, which is considered boolean, to control that. When set to True, scan_repo_files won't do anything and thus avoid above described situation. As in most cases fdroidserver is used for apps only, the example config.py has the option set to true.