Update Twitter URLs to use a new URI that doesn't mention twitter.com

[afandian]

Per new regulations at https://developer.twitter.com/en/developer-terms/more-on-restricted-use-cases we may only include the Tweet and author id, not the URL to the tweet. As we need to represent the subject ID as at least a URI, we'll need a way of unambiguously representing the ID.

Components affected:

• event-data-agents - the generation of the URL
• event-data-investigator - twitter compliance must be able to extract the URL correctly
• event-data-user-guide - document new behaviour