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/shared/search/code/results.tmpl

@@ -11,7 +11,7 @@
 <div class="repository search">
 	{{range $result := .SearchResults}}
 		{{$repo := or $.Repo (index $.RepoMaps .RepoID)}}
-		<div class="diff-file-box diff-box file-content non-diff-file-content repo-search-result">
+		<div class="diff-file-box file-content non-diff-file-content repo-search-result">
 			<h4 class="ui top attached header tw-font-normal tw-flex tw-flex-wrap">
 				{{if not $.Repo}}
 					<span class="file tw-flex-1">