[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
if versions_map['cubicweb'][0] == (3, 7, 4):
config['http-session-time'] *= 60
config['cleanup-session-time'] *= 60
config['cleanup-anonymous-session-time'] *= 60