Separate Blockcerts contexts into Blockcerts core and OBI extensions

[kimdhamilton]

This tracks separating Blockcerts definitions specific to OBI contexts vs core Blockcerts, allowing Blockcert verification to be used in different domains, where Open Badges vocabulary may not be relevant.

Blockcerts' OBI-related vocabulary will be expressed as an OBI extension where appropriate/possible, and the rest can be declared in a Blockcerts/OBI JSON-LD context. Those are tracked in #28

Blockcerts core definitions are those related to signing and verification. While these will be expressed as Open Badge Extensions, Blockcerts should also define these core types for use across domains.

signature will look structurally the same as defined in #25.

The elements required for verification, on the other hand, is spread out in an OBI certificate. A unified verification block requires type(s) for storing:

• Verification type
• Issuer identification info, such as issuer public key
• Revocation info
• Recipient public key
• Expected signature suite

[ottonomy]

"issuedOn" is still the same property name. I had an error in one of the new examples.

However, OBI timestamps must now include time and a time zone indicator in ISO-8601.

[kimdhamilton]

excellent -- we'd already specified ISO-8601 in the blockcerts schema, so now we can just reuse the OBI definition

[kimdhamilton]

Merged