CORS endpoints support

[bigcat88]

resolves #80

Requires modification in the Nextcloud core(CORSMiddleware.php):

After passesCSRFCheck we add skip condition based on value set in AppAPI authentication.

// Skip CORS check for requests with AppAPI auth.
if ($this->session->get('app_api') === true) {
    return;
}

[bigcat88]

CI errors unrelated to this PR, after we merge #77 (today/tomorrow) they will gone.