Allow collaborators to request branches they are allowlisted in
    
    If user or group collaborators are granted access to, e.g. "epel*",
    they should be able to request branches that match this globbing, e.g.
    "epel9".
    
    Pagure has an API endpoint for reporting contributors in the level
    of detail we need; add a wrapper in `pagure.py` (`get_project_contributors`)
    and use it in `utils.py`'s `prompt_for_new_branch`
    
    Signed-off-by: Michel Alexandre Salim <salimma@fedoraproject.org>