Configurable KeyServers List

[h3cksamrat]

It would be great if Docker users could configure the PGP public keyserver hosts directly from the admin panel.
Currently, users who build from source can edit the keyservers.php file to specify which keyservers to use. However, Docker users and those running pre-built images don’t have an easy way to change keyserver hosts without modifying source files or rebuilding the image. Making the keyserver host configurable through the admin panel would simplify this process and improve usability for all users, especially those deploying SnappyMail via Docker.
This change would allow administrators to quickly adapt to issues with public keyservers or switch to private keyservers as needed, all without touching the underlying files or rebuilding containers.