HackMD ORN Implementation

[lukvmil]

Working on HackMD ORN implementation in the hackmd branch. Current proposed implementation looks like orn:hackmd.note:<team_path>/<note_id>. There are actually multiple ways to access notes and the note id is globally unique. The API recommends calling api.hackmd.io/v1/teams/<team_path>/notes/<note_id> but you can actually just call api.hackmd.io/v1/notes/<note_id>. In the case of referencing public notes, you cannot use the team based route, so maybe better to simply use the note id? Metadata only tells you the user/team that owns the note, and the last user that edited it. Do we have a need for hackmd.team/user objects?