Authenticate an app

        :param app_id: the app id
        :param app_secret: the app secret
        :param auth_code: the app auth code

def auth_app(self, app_id, app_secret, auth_code, state=''):
        """
        Authenticate an app

        :param app_id: the app id
        :param app_secret: the app secret
        :param auth_code: the app auth code
        """
        headers = {
            'Content-type': 'application/json'
        }
        payload = {
            'application': app_id,
            'auth_code': auth_code,
            'state': state
        }
        try:
            full_url = utils.urljoin(
                self.host,
                '/api/v1/application-tokens/validate'
            )
            response = requests.post(
                full_url,
                data=json.dumps(payload),
                headers=headers,
                verify=self.tls_verify
            )
        except RequestException:
            raise exceptions.TaigaRestException(
                full_url, 400,
                'NETWORK ERROR', 'POST'
            )
        if response.status_code != 200:
            raise exceptions.TaigaRestException(
                full_url,
                response.status_code,
                response.text,
                'POST'
            )
        cyphered_token = response.json().get('cyphered_token', '')
        if cyphered_token:
            from jwkest.jwk import SYMKey
            from jwkest.jwe import JWE

            sym_key = SYMKey(key=app_secret, alg='A128KW')
            data, success = JWE().decrypt(cyphered_token, keys=[sym_key]), True
            if isinstance(data, tuple):
                data, success = data
            try:
                self.token = json.loads(data.decode('utf-8')).get('token', None)
            except ValueError:  # pragma: no cover
                self.token = None
            if not success:
                self.token = None
        else:
            self.token = None

        if self.token is None:
            raise exceptions.TaigaRestException(
                full_url, 400,
                'INVALID TOKEN', 'POST'
            )

        self.raw_request = RequestMaker('/api/v1', self.host, self.token,
                                        'Application', self.tls_verify)
        self._init_resources()