cubicweb-email #2438204 make MBOXImporter extensible [validation pending]

MBOXImporter could be reused elsewhere if only it returned the import thread eids.

prioritynormal
typeenhancement
done in1.9.0
load0.100
load left0.000
closed by#16116009a01f [mboximport] refactor mboximport to make it easier to subclass / customize (closes #2438204)