[MM-41785] Show error in invitation popin when email can't be send (#19617)

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/server.go

@@ -125,7 +125,7 @@ type Server struct {
 	goroutineCount      int32
 	goroutineExitSignal chan struct{}
 
-	EmailService *email.Service
+	EmailService email.ServiceInterface
 
 	hubs     []*Hub
 	hashSeed maphash.Seed