BibIndex: new `datasource` field and `786__w` tag

* Adds field `datasource`, associated with the new tag `786__w`

* Adds upgrade script

* Adds bibfield definition of `datasource` in `atlantis.cfg`

* Sets CFG_HEPDATA_INDEX and CFG_HEPDATA_FIELD to `datasource`

Signed-off-by: Federico Poli <federico.poli@cern.ch>