Add link to user profile in markdown mention only if user exists (#21533)

Previously mentioning a user would link to its profile, regardless of
whether the user existed. This change tests if the user exists and only
if it does - a link to its profile is added.

* Fixes #3444

Signed-off-by: Yarden Shoham <hrsi88@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

services/markup/processorhelper.go

@@ -0,0 +1,29 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package markup
+
+import (
+	"context"
+
+	"code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/markup"
+)
+
+func ProcessorHelper() *markup.ProcessorHelper {
+	return &markup.ProcessorHelper{
+		IsUsernameMentionable: func(ctx context.Context, username string) bool {
+			// TODO: cast ctx to modules/context.Context and use IsUserVisibleToViewer
+
+			// Only link if the user actually exists
+			userExists, err := user.IsUserExist(ctx, 0, username)
+			if err != nil {
+				log.Error("Failed to validate user in mention %q exists, assuming it does", username)
+				userExists = true
+			}
+			return userExists
+		},
+	}
+}