API: registering an app id and uploading an app release needs to check revoked certificates

[BernhardPosselt]

In order to prevent old or lost certificates from being abused we need to check if the certificate has been revoked. This has to be done before validating the certificate on app release upload and before registering a new app id.

[BernhardPosselt]

@LukasReschke need the following:

• Empty original Nextcloud CRL file in PEM format: https://github.com/nextcloud/appstore/blob/master/nextcloudappstore/core/certificate/nextcloud.crl

Then generate a new CA (e.g. imaginary) and place the following files here:

• The chain file: https://github.com/nextcloud/appstore/tree/master/nextcloudappstore/core/certificate/tests/data/certificates/imaginary.chain
• A valid and signed cert here: https://github.com/nextcloud/appstore/tree/master/nextcloudappstore/core/certificate/tests/data/certificates/imaginary-valid.crt
• Two valid and but revoked certs here: https://github.com/nextcloud/appstore/tree/master/nextcloudappstore/core/certificate/tests/data/certificates/imaginary-revoked1.crt, https://github.com/nextcloud/appstore/tree/master/nextcloudappstore/core/certificate/tests/data/certificates/imaginary-revoked2.crt
• The CRL file with the two revoked certs here: https://github.com/nextcloud/appstore/tree/master/nextcloudappstore/core/certificate/tests/data/certificates/imaginary.crl