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/add_reaction.tmpl

@@ -3,7 +3,7 @@
 	<a class="muted">{{svg "octicon-smiley"}}</a>
 	<div class="menu">
 		{{range $value := AllowedReactions}}
-		<a class="item emoji comment-reaction-button" data-tooltip-content="{{$value}}" aria-label="{{$value}}" data-reaction-content="{{$value}}">{{ReactionToEmoji $value}}</a>
+		<a class="item emoji" data-tooltip-content="{{$value}}" aria-label="{{$value}}" data-reaction-content="{{$value}}" data-global-click="onCommentReactionButtonClick">{{ReactionToEmoji $value}}</a>
 		{{end}}
 	</div>
 </div>