Vocabulary and Modeling: Add 'Grant' activity and related properties (#156)
    
    Adding a new activity type `Grant`, used for granting access to shared
    resources (repos, ticket trackers, etc.) and some related proeprties:
    
    - `fulfills`, for automatic access delegations to specify which manually
      sent Grant triggered them (allows to group grants in UI, display them
      in a way that makes sense to human users, and avoid unmanageable mess)
    - `capability`, for activities to provide authentication by specifying a
      previously published `Grant` as proof of access (it's called
      capability because it's an Object Capability)
    
    I'm not adding `Grant` to the Behavior spec yet because the explanation
    will rely on types that haven't been added yet, such as `TicketTracker`.
    I'll add them in a separate PR and then I'll add a Behavior section
    for `Grant`
    
    Co-authored-by: fr33domlover <fr33domlover@riseup.net>
    Reviewed-on: https://codeberg.org/ForgeFed/ForgeFed/pulls/156
    Reviewed-by: Anthony Wang <xy@noreply.codeberg.org>
    Co-authored-by: fr33domlover <fr33domlover@noreply.codeberg.org>
    Co-committed-by: fr33domlover <fr33domlover@noreply.codeberg.org>