Fix dynamic content loading init problem (#33748)

1. Rewrite `dirauto.ts` to `observer.ts`. 
* We have been using MutationObserver for long time, it's proven that it
is quite performant.
    * Now we extend its ability to handle more "init" works.
2. Use `observeAddedElement` to init all non-custom "dropdown".
3. Use `data-global-click` to handle click events from dynamically
loaded elements.
* By this new approach, the old fragile selector-based
(`.comment-reaction-button`) mechanism is removed.
4. By the way, remove unused `.diff-box` selector, it was abused and
never really used.

A lot of FIXMEs in "repo-diff.ts" are completely fixed, newly loaded
contents could work as expected.

templates/repo/issue/view_content/conversation.tmpl

@@ -40,7 +40,7 @@
 		{{if $diff}}
 			{{$file := (index $diff.Files 0)}}
 			<div id="code-preview-{{$comment.ID}}" class="ui table segment{{if $resolved}} tw-hidden{{end}}">
-				<div class="diff-file-box diff-box file-content {{TabSizeClass $.Editorconfig $file.Name}}">
+				<div class="diff-file-box file-content {{TabSizeClass $.Editorconfig $file.Name}}">
 					<div class="file-body file-code code-view code-diff code-diff-unified unicode-escaped">
 						<table>
 							<tbody>