[server] Move "system" source initialization into its own method

[entities] Make CWSource's dictconfig method clearer

[entities] Move CWSource update_config method as a function in 3.15 migration script This script is the only caller of this method. In the code of the function, drop "skip_unknown" argument and continue accordingly upon OptionError.

[server/test] Turn "pull" method of LDAPFeedTestBase into a staticmethod Because it does not use reference to "self" (which should be "cls" for classmethod).