Allow external certificates to be uploaded when hosted on Kubernetes

[rudivanhierden]

Currently, all sites receive certificates through LetsEncrypt by default (for the kubernetes version at least). In some cases it's preferable to use an external certificate, for instance issues by QuoVadis or Cloudflare.

It's not possible to upload such a certificate through the admin panel, and it now needs to be added to the kubernetes secrets manually. A good short-term situation would be:

• A private key + certificate (including the correct chain) can be uploaded through the admin panel
• The key gets checked through the x509 module (https://www.npmjs.com/package/x509), so invalid certificates can't be uploaded
• The valid certificate gets saved as a secret and added to the site's ingress
• The certificate's expiration date is saved so we can alert the admin before expiration