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>

templates/repo/editor/commit_form.tmpl

@@ -22,7 +22,7 @@
 				<label>{{ctx.Locale.Tr "repo.editor.signoff_desc"}}</label>
 			</div>
 		</div>
-		<div class="quick-pull-choice js-quick-pull-choice">
+		<div class="field quick-pull-choice js-quick-pull-choice">
 			<div class="field">
 				<div class="ui radio checkbox {{if not .CommitFormOptions.CanCommitToBranch}}disabled{{end}}">
 					<input type="radio" class="js-quick-pull-choice-option" name="commit_choice" value="direct" data-button-text="{{ctx.Locale.Tr "repo.editor.commit_changes"}}" {{if eq .commit_choice "direct"}}checked{{end}}>
@@ -30,9 +30,9 @@
 						{{svg "octicon-git-commit"}}
 						{{ctx.Locale.Tr "repo.editor.commit_directly_to_this_branch" .BranchName}}
 						{{if not .CommitFormOptions.CanCommitToBranch}}
-						<div class="ui visible small warning message">
+						<div class="tw-mt-2">
 							{{ctx.Locale.Tr "repo.editor.no_commit_to_branch"}}
-							<ul>
+							<ul class="tw-mb-0">
 								{{if not .CommitFormOptions.UserCanPush}}<li>{{ctx.Locale.Tr "repo.editor.user_no_push_to_branch"}}</li>{{end}}
 								{{if and .CommitFormOptions.RequireSigned (not .CommitFormOptions.WillSign)}}<li>{{ctx.Locale.Tr "repo.editor.require_signed_commit"}}</li>{{end}}
 							</ul>