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

#### 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`

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#### Miscutil: new demo records

* Adds 4 demo records
* Fix tests

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Signed-off-by: Federico Poli <federico.poli@cern.ch>