nixos/zammad: refactor package, module and nixos-test

[NetaliDev]

Description of changes

This PR refactors the Zammad package, NixOS module and NixOS test. It does the following things:

• Use inherit (lib) ... instead of with lib;
• Make the Zammad system user and group configurable
• Use symlinks in the package to prevent the content of the package being copied to the state directory at every start of the systemd unit
• Drop MySQL support for multiple reasons:
  • It wasn't really supported by the module before, since the zammad-web unit hard depended on a local PostgreSQL database (especially in the check if a database initialization is required)
  • Support for MySQL will be dropped in Zammad 7 (see here)
• Replace complicated, PostgreSQL specific query to check if the database was already seeded with a simple file in the state directory
  • This also enables compatibility with other DB Backends that may be supported in the future
• Fix the systemd unit dependencies so that it only depends on a local PostgreSQL DB if it is created locally
• Remove locale hack in the test since it is not required anymore
• Remove hardcoded debug log level
• Add a cleanup task to the preScript of the zammad-web unit to clean the old package contents out of the state directory

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[NetaliDev]

@ofborg build zammad
@ofborg test zammad

[Radvendii]

This is a clever change. I like that it leaves more things in the nix store rather than copied into runtime directories.

[Radvendii]

I haven't tested it, but the code looks good to me.

[mweinelt]

Did a rebase, since what was there conflicted in a few places.

@Radvendii PTAL and ping me to merge.