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.

web_src/css/repo.css

@@ -1085,10 +1085,6 @@ td .commit-summary {
   height: 30px;
 }
 
-.repository .diff-box .resolved-placeholder .button {
-  padding: 8px 12px;
-}
-
 .repository .diff-file-box .header {
   background-color: var(--color-box-header);
 }