Add the EXCLUDE_GROUP_INDEX feature and corresponding tests
    
    This new configuration key allows to hide projects an user has only
    access to via one or more specified group. This will not remove the
    access, just not show the project on the user's user page in order
    to not show the 17,000 projects that the people in the provenpackager
    group in Fedora have access to when pagure is used as a front-end
    for Fedora's git repositories.