Fix the new PR drop-down button
    
    Currently we only populate the new PR drop-down button if the user
    is a maintainer, however, if the user has a fork, the drop-down
    button should work.
    So instead of preventing the entire button from working, we've
    adjusted so that only the main project is considered if the user
    is a maintainer and the fork is considered everytime otherwise.
    
    Fixes https://pagure.io/pagure/issue/4310
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>