[sources] Check source's url attribute value on creation/modification Similarly as for config. Now, ldap source validation is properly done there instead of at initialization time.

[sources] Check sources configuration is fine on creation/modification Reintroduce usage of 'source.check_config' which had almost disappeared, as well as tests for the currently detected errors. Part of the system source specific checking done in syncsources reimplemented in a specific check_config implementation. Tests are dispatched among ldap / datafeed and syncsources tests but are not strictly correctly located (notably syncsources tests behaviour of the native source's check_config). The system source url checking part which disappears from syncsources will be reintroduced in a follow-up.