Allow hard deletes for compliance

[afandian]

Compliance agreements e.g. from Twitter require that we remove Twitter data from Events.

NB: This is different to deletes in e.g. Wikipedia where this is an extra event. Twitter ToC specifies that the data should be actually deleted.

When an Agent sends a DELETE for an Event ID, the operation should succeed when:

• when the claims in the token authenticate the agent
• the event ID exists
• the event source id and source token match those claimed by the Agent

A delete operation should:

• update the Event in the original log
• update the Event in the archive
• re-send the updated Event via outbound interfaces

A delete operation changes an Event in the following ways:

• remove the Source ID. Should be changed to a blank node like https://eventdata.crossref.org/deleted
• remove the Subject Metadata such that there is no external data in there.

A delete operation should not:

• delete the actual event or its ID