refactor: clean ui modal component (#39654)

1. use "button" element for the modal close button
2. remove "ui modal" width hacks

templates/devtest/fomantic-modal.tmpl

@@ -64,7 +64,7 @@
 	</div>
 
 	<div class="ui g-modal-confirm modal" id="test-modal-danger">
-		<div class="header">dangerous action dialog {{svg "octicon-x" 16 "close-modal"}}</div>
+		<div class="header">dangerous action dialog <button type="button" class="close-modal"></button></div>
 		<div class="content">hello, this is the modal dialog content, this is a dangerous operation</div>
 		{{template "base/modal_actions_confirm" (dict "ModalButtonDangerText" "I know and must do  this is dangerous operation")}}
 	</div>