CC schema/example update proposal (#112)

Addresses issues:
#100 #91 #47 

Changes:

- Move `issuedBy` to `issuer` field of VC
- Remove `issuedTo` and instead have the `credentialSubject` be the
issued to entity and shift the attestation into a `hasAttestation`
property, similar to https://schema.org/hasCertification.
- Move the evidence field of the `ConformityAttestation` to the evidence
field of the VC, defining an evidence type of
`ConformityAttestationEvidence`.
- Delegate the `status` field of the `ConformityAttestation` to the
`credentialStatus` component of the VCDM (BitstringStatusList).
- Replace the concept of the facility of the assessment with a location.
- Replace terms `subjectProducts` and `subjectFacilities` with
`attestedProducts` and `attestedLocations`
- Add new UNTP context file

I will keep this PR scoped to this, most of it addresses the alignment
with the core data model and some of it addresses some conceptual
changes.

Looking for comments and feedback here please! Most of these changes
came from our exploration with the `MinesActPermit` and the
`TenureTitle` `ConformityCredentials` over at BC Gov.