Verify tarball checksum from Appstore when downloading app release

[provokateurin]

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Is your feature request related to a problem? Please describe.
Releases are not stored on the Appstore but on 3rd party services. In case of security issues an attacker might replace the release tarball.

Describe the solution you'd like
When downloading a release on the server the checksum from the Appstore should be used to verify the release.

Appstore counter-part: nextcloud/appstore#1499