[REF] Move DBMgr to indico.core.db

- Includes in the old location the new location
[REF] Moved core fossils.py to indico.core.fossils
[REF] classFactory method with dictionaries

- Closes #1385
[IMP] Check better the dictionaries

- Added also documentation
[DEL] Deleted MaKaC.common.db

- See #1385