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>