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

* Adds field `datasource`, associated with the new tag `786__w`
* Adds upgrade script

Signed-off-by: Federico Poli <federico.poli@cern.ch>
1: D100 Docstring missing
25: D103 Docstring missing
29: D103 Docstring missing
33: D103 Docstring missing
46: D103 Docstring missing
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54: D103 Docstring missing