[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.

doc/book usr/share/doc/cubicweb-documentation
doc/html usr/share/doc/cubicweb-documentation
debian/cubicweb-doc usr/share/doc-base/cubicweb-doc