Fix various problems (#37547)

1. Fix ugly commit form "warning" message
2. Use JSONError for "Update PR Branch" response 
3. Remove useless "timeline" class
4. Make timeline review default to "comment" to avoid icon missing
5. Align PR's "command line instructions" UI
6. Simply "Update PR branch" button logic

And then some TODOs are fixed.

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>

tests/e2e/reactions.test.ts

@@ -12,7 +12,7 @@ test('toggle issue reactions', async ({page, request}) => {
   ]);
   await page.goto(`/${owner}/${repoName}/issues/1`);
 
-  const issueComment = page.locator('.timeline-item.comment.first');
+  const issueComment = page.locator('.timeline-item.comment.issue-content-comment');
 
   const reactionPicker = issueComment.locator('.select-reaction');
   await reactionPicker.click();