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/renderer.go

@@ -19,8 +19,18 @@ import (
 	"code.gitea.io/gitea/modules/setting"
 )
 
+type ProcessorHelper struct {
+	IsUsernameMentionable func(ctx context.Context, username string) bool
+}
+
+var processorHelper ProcessorHelper
+
 // Init initialize regexps for markdown parsing
-func Init() {
+func Init(ph *ProcessorHelper) {
+	if ph != nil {
+		processorHelper = *ph
+	}
+
 	NewSanitizer()
 	if len(setting.Markdown.CustomURLSchemes) > 0 {
 		CustomLinkURLSchemes(setting.Markdown.CustomURLSchemes)