Automate package release #64

Rather than manually having to release packages to the Nextcloud App Store, this should be automated and run for each tagged release.

How to setup this automation can be gleaned from these sources:

• https://help.nextcloud.com/t/app-release-process/73984
• https://blog.wuc.me/2018/08/14/nextcloud-app-release-process.html

And by looking at the setup in other apps (for instance user_oidc and mail).