Make the group_name be of max 255 characters
    
    For some reasons we were limiting them to 16 chars, while FAS uses 32
    and since pagure does not per say require FAS to work, there is no
    good reason to use 32 rather than 255, so this commit bumps the limit
    directly to 255.
    
    Fixes https://pagure.io/pagure/issue/1616