generate-config subcommand for the main binary

[aumetra]

This would start a small interactive questionaire about the basic configuration of the instance.
Such as "What's the domain?", "Do you use TLS?", "What's the instance name?", "What's the database URL?", etc. and then generates a functional configuration based on an Askama template embedded into the binary.

To ensure the configuration is valid, we should take our configuration and add a JSON schema check (or just deserialize a freshly generated configuration against the most up-to-date struct).