[config] cleanup/refactor server sources file values handling * kill former `sources` method, misnamed since we've only the system source configuration in the sources file (and some default admin account information) * introduce new system_source_config and default_admin_config properties to access to the two information in this file Then use one or the other when appropriate

[repoapi] drop get_option_value's foreid argument repoapi was introduced in 3.19, so no need to introduce legacy deprecated stuff at the same time.