Add sub issue list support (#32940)

Just like GitHub, show issue icon/title when the issue number is in a list

services/markup/renderhelper.go

@@ -0,0 +1,34 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package markup
+
+import (
+	"context"
+
+	"code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/markup"
+	gitea_context "code.gitea.io/gitea/services/context"
+)
+
+func FormalRenderHelperFuncs() *markup.RenderHelperFuncs {
+	return &markup.RenderHelperFuncs{
+		RenderRepoFileCodePreview: renderRepoFileCodePreview,
+		RenderRepoIssueIconTitle:  renderRepoIssueIconTitle,
+		IsUsernameMentionable: func(ctx context.Context, username string) bool {
+			mentionedUser, err := user.GetUserByName(ctx, username)
+			if err != nil {
+				return false
+			}
+
+			giteaCtx, ok := ctx.(*gitea_context.Context)
+			if !ok {
+				// when using general context, use user's visibility to check
+				return mentionedUser.Visibility.IsPublic()
+			}
+
+			// when using gitea context (web context), use user's visibility and user's permission to check
+			return user.IsUserVisibleToViewer(giteaCtx, mentionedUser, giteaCtx.Doer)
+		},
+	}
+}