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>

routers/api/v1/misc/markdown_test.go

@@ -5,6 +5,7 @@
 package misc
 
 import (
+	go_context "context"
 	"io"
 	"net/http"
 	"net/http/httptest"
@@ -13,6 +14,7 @@ import (
 	"testing"
 
 	"code.gitea.io/gitea/modules/context"
+	"code.gitea.io/gitea/modules/markup"
 	"code.gitea.io/gitea/modules/setting"
 	api "code.gitea.io/gitea/modules/structs"
 	"code.gitea.io/gitea/modules/templates"
@@ -50,6 +52,11 @@ func wrap(ctx *context.Context) *context.APIContext {
 
 func TestAPI_RenderGFM(t *testing.T) {
 	setting.AppURL = AppURL
+	markup.Init(&markup.ProcessorHelper{
+		IsUsernameMentionable: func(ctx go_context.Context, username string) bool {
+			return username == "r-lyeh"
+		},
+	})
 
 	options := api.MarkdownOption{
 		Mode:    "gfm",