REST endpoints, associated Porter Learner functions, and think about failure HTTP codes.

Draft API provided by #2666 . Can re-iterate here if required, but I'll just summarize - see #2666 for full context.
For Alice::grant:

• /get_ursulas - Scope Porter functionality needed by Alice #2666 (comment)
• /publish_treasure_map - Scope Porter functionality needed by Alice #2666 (comment)

For Alice::revoke

• /revoke - Scope Porter functionality needed by Alice #2666 (comment)