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>

routers/web/repo/pull.go

@@ -974,17 +974,15 @@ func UpdatePullRequest(ctx *context.Context) {
 		return
 	}
 
-	// ToDo: add check if maintainers are allowed to change branch ... (need migration & co)
 	if (!allowedUpdateByMerge && !rebase) || (rebase && !allowedUpdateByRebase) {
-		ctx.Flash.Error(ctx.Tr("repo.pulls.update_not_allowed"))
-		ctx.Redirect(issue.Link())
+		ctx.JSONError(ctx.Tr("repo.pulls.update_not_allowed"))
 		return
 	}
 
 	// default merge commit message
 	message := fmt.Sprintf("Merge branch '%s' into %s", issue.PullRequest.BaseBranch, issue.PullRequest.HeadBranch)
 
-	// The update process should not be cancelled by the user
+	// The update process should not be canceled by the user
 	// so we set the context to be a background context
 	if err = pull_service.Update(graceful.GetManager().ShutdownContext(), issue.PullRequest, ctx.Doer, message, rebase); err != nil {
 		if pull_service.IsErrMergeConflicts(err) {
@@ -998,8 +996,7 @@ func UpdatePullRequest(ctx *context.Context) {
 				ctx.ServerError("UpdatePullRequest.HTMLString", err)
 				return
 			}
-			ctx.Flash.Error(flashError)
-			ctx.Redirect(issue.Link())
+			ctx.JSONError(flashError)
 			return
 		} else if pull_service.IsErrRebaseConflicts(err) {
 			conflictError := err.(pull_service.ErrRebaseConflicts)
@@ -1012,19 +1009,18 @@ func UpdatePullRequest(ctx *context.Context) {
 				ctx.ServerError("UpdatePullRequest.HTMLString", err)
 				return
 			}
-			ctx.Flash.Error(flashError)
-			ctx.Redirect(issue.Link())
+			ctx.JSONError(flashError)
 			return
 		}
-		ctx.Flash.Error(err.Error())
-		ctx.Redirect(issue.Link())
+		log.Error("Update pull request failed: %v", err)
+		ctx.JSONError("Unable to update pull request")
 		return
 	}
 
-	time.Sleep(1 * time.Second)
+	time.Sleep(100 * time.Millisecond) // TODO: it is really questionable whether the Sleep is useful here, need to figure out
 
 	ctx.Flash.Success(ctx.Tr("repo.pulls.update_branch_success"))
-	ctx.Redirect(issue.Link())
+	ctx.JSONRedirect(issue.Link())
 }
 
 // MergePullRequest response for merging pull request