Send registration email on user autoregistration (#16523)

When users login and are autoregistered send email notification.

Fix #16178

* Protect public functions within the mailer by testing if the mailer is configured

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: 6543 <6543@obermui.de>

services/mailer/mail_release.go

@@ -23,6 +23,11 @@ const (
 
 // MailNewRelease send new release notify to all all repo watchers.
 func MailNewRelease(rel *models.Release) {
+	if setting.MailService == nil {
+		// No mail service configured
+		return
+	}
+
 	watcherIDList, err := models.GetRepoWatchersIDs(rel.RepoID)
 	if err != nil {
 		log.Error("GetRepoWatchersIDs(%d): %v", rel.RepoID, err)