Add icon to buttons "Close with Comment", "Close Pull Request", "Close Issue" (#36654)

Newbies often use the "Close with Comments" button instead of deleting
their comment. Icon should prevent mis-clicks.

---------

Co-authored-by: Yuriy.Khlynovskiy <[email protected]>
Co-authored-by: silverwind <[email protected]>
Co-authored-by: Claude Opus 4.6 <[email protected]>
Co-authored-by: wxiaoguang <[email protected]>

web_src/css/base.css

@@ -1095,7 +1095,6 @@ table th[data-sortt-desc] .svg {
 }
 
 .btn,
-.ui.ui.button,
 .ui.ui.dropdown,
 .flex-text-inline,
 .flex-text-inline > a,