Gravatar fallback does not respect 'AppSubUrl'-setting (#12149)

Closes #12148

pkg/api/dtos/models.go

@@ -52,7 +52,7 @@ type UserStars struct {
 
 func GetGravatarUrl(text string) string {
 	if setting.DisableGravatar {
-		return "/public/img/user_profile.png"
+		return setting.AppSubUrl + "/public/img/user_profile.png"
 	}
 
 	if text == "" {