[workflow test] don't use session.user, subject to internal changes

[db creation test] more testing of db initialization: call build_db_cache and ensure admin user exists

[source synchronization hooks] Fix/enhance system source hooks They are broken if launched during repository initialization (this was not the case yet, but will be soon...). Add additional checks to ensure one doesn't try to store system source config in the database, as it will be ignored in favor of the "sources" file.