Use WebID as record identifier

[elf-pavlik]

As discussed in #16 and matrix chat. This PR includes in separate commits:

• added migration script
• migrated data
• added validation script to ensure id always equals webid

I will create a separate issue to refactor scripts handling data since it starts getting messy and could use some DRY #70

There was a duplicate record:

cdata:Pat_McBennett a ex:Person ;
  ex:landingPage <https://github.com/pmcb55> ;
  ex:name "Pat McBennett" ;
  ex:webid <https://pmcb55.inrupt.net/profile/card#me> .

cdata:Patrick a ex:Person ;
  ex:name "Patrick" ;
  ex:webid <https://pmcb55.inrupt.net/profile/card#me> .

Which got merged using only Pat McBennett name.

[elf-pavlik]

Suggested change

	dataset.delete(quad)
	dataset.add(changeSubject(quad, webid))

This could be removed since it will match on subject anyways and be taken care of there.
At the same time it is one-time migration script which has no tests so maybe better not to touch it for such a minor tiding up.