Gitea postgres autocreate database

[etu]

Motivation for this change

Before these changes it was required to enable postgres yourself and set up a role and everything to be able to use it.

After these changes it does all of this for you:

  # Gitea
  services.gitea.enable = true;
  services.gitea.appName = "My Git Service";
  services.gitea.rootUrl = "http://git2.example.com/";
  services.gitea.database.type = "postgres";
  services.gitea.database.password = "this-is-my-database-password";

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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[etu]

cc @disassembler maybe?