convert Stud.IP username (LUH-ID) to lowercase

controllers/matrix_chat.php

@@ -61,7 +61,7 @@ class MatrixChatController extends AuthenticatedController
             if (!$this->account && !Config::get()->MATRIX_AUTO_CREATE_ACCOUNTS) {
                 $this->account = new MatrixAccount();
                 $this->account->user_id = User::findCurrent()->id;
-                $this->account->matrix_account_id = '@' . User::findCurrent()->username . ':' .
+                $this->account->matrix_account_id = '@' . strtolower(User::findCurrent()->username) . ':' .
                     Config::get()->MATRIX_ACCOUNTS_LOCAL_PART;
                 $this->account->matrix_password = '';
                 $this->account->store();