Fetch API token on app upload

[janikarki]

Currently on the app upload page the API token is fetched on page load. It should instead be fetched if and when the form is submitted. This would allow the form to function properly even if the token is regenerated between the page load and the submission.

Token fetching and form submission is done in https://github.com/nextcloud/appstore/blob/master/nextcloudappstore/core/static/assets/js/app-upload.js using functions defined in https://github.com/nextcloud/appstore/blob/master/nextcloudappstore/core/static/assets/js/utils.js.