Add a modal at the bottom of the issues list to add milestones
    
    With this, from the list of issues one can now directly add milestones
    (if they are admin of course).
    This will then redirect them back to the first page of the issue list.
    
    Fixes #2472
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>