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>

modules/markup/markdown/markdown_test.go

@@ -38,6 +38,11 @@ func TestMain(m *testing.M) {
 	if err := git.InitSimple(context.Background()); err != nil {
 		log.Fatal("git init failed, err: %v", err)
 	}
+	markup.Init(&markup.ProcessorHelper{
+		IsUsernameMentionable: func(ctx context.Context, username string) bool {
+			return username == "r-lyeh"
+		},
+	})
 	os.Exit(m.Run())
 }