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>

options/locale/locale_en-US.json

@@ -1306,7 +1306,7 @@
   "repo.editor.upload_file_is_locked": "File \"%s\" is locked by %s.",
   "repo.editor.upload_files_to_dir": "Upload files to \"%s\"",
   "repo.editor.cannot_commit_to_protected_branch": "Cannot commit to protected branch \"%s\".",
-  "repo.editor.no_commit_to_branch": "Unable to commit directly to branch because:",
+  "repo.editor.no_commit_to_branch": "Not allowed to commit directly to branch because:",
   "repo.editor.user_no_push_to_branch": "User cannot push to branch",
   "repo.editor.require_signed_commit": "Branch requires a signed commit",
   "repo.editor.cherry_pick": "Cherry-pick %s onto:",