Improve user experience for outdated comments (#29050)

Try to improve #28949

1. Make `ctx.Data["ShowOutdatedComments"] = true` by default: it brings
consistent user experience, and sometimes the "outdated (source
changed)" comments are still valuable.
2. Show a friendly message if the comment won't show, then the end users
won't fell that "the comment disappears" (it is the special case when
`ShowOutdatedComments = false`)

routers/web/repo/pull_review.go

@@ -22,6 +22,7 @@ import (
 
 const (
 	tplDiffConversation     base.TplName = "repo/diff/conversation"
+	tplConversationOutdated base.TplName = "repo/diff/conversation_outdated"
 	tplTimelineConversation base.TplName = "repo/issue/view_content/conversation"
 	tplNewComment           base.TplName = "repo/diff/new_comment"
 )
@@ -161,8 +162,8 @@ func renderConversation(ctx *context.Context, comment *issues_model.Comment, ori
 		return
 	}
 	if len(comments) == 0 {
-		// if the comments are empty (deleted, outdated, etc), it doesn't need to render anything, just return an empty body to replace "conversation-holder" on the page
-		ctx.Resp.WriteHeader(http.StatusOK)
+		// if the comments are empty (deleted, outdated, etc), it's better to tell the users that it is outdated
+		ctx.HTML(http.StatusOK, tplConversationOutdated)
 		return
 	}