Update the Preview button to display 'Edit' when previewing
    
    While adjusting the label of the Preview button rework the pull-request
    template to reduce code duplication and be able to use the same piece of
    code when adding comments as when opening a new PR.
    
    Fixes https://pagure.io/pagure/issue/1781
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>