Behavior: Support `Reject`ing a Join request (#162)
    
    If Bob asks for access to MyProject, possibly Alice approves using an
        Accept and then MyProject sends a Grant. But what if Alice doesn't trust
        Bob? Until now, her only option was:
    
        1. Alice sees the Join that Bob sent, and decides not to send an Accept
        2. Celine sees the Join, doesn't know that Alice doesn't trust Bob, and
           decides to send an Accept
        3. MyProject sends a Grant
        4. Alice notices what happened and is now surprised and upset...
    
        This commit gives Alice a better option:
    
        1. Alice sees the Join, and decides to sent a Reject
        2. If Celine doesn't notice the Reject and sends an Accept, MyProject
           refuses to send a Grant because it considers the Join already
           rejected by Alice
        3. Alice, Bob and Celine can now talk and figure out if/how Bob can join
           and contribute
    
    Co-authored-by: fr33domlover <fr33domlover@riseup.net>
    Reviewed-on: https://codeberg.org/ForgeFed/ForgeFed/pulls/162
    Reviewed-by: Anthony Wang <xy@noreply.codeberg.org>