Allow linking to the new issue page with a specific template
    
    With this commit we can construct link to the new_issue page with
    a ?template argument taking the name of the template file on disk
    and show the new_page with that template in.
    
    Fixes https://pagure.io/pagure/issue/3759
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>