Motivation for this change

Installing Redmine with plugins. However I don't think it's reasonably doable to let the users select the wanted plugins and still ensuring reproducibility by fixing a gemset.nix, that would amount to roughly 2^|plugins| gemset.nix files.
I'm just sharing this attempt as an experiment that should not be merged, but might inspire others who know Nix better than I do. In the end, I myself am not sure to need Redmine, and the configuration of the main plugin I wanted is too difficult: I was unable to understand why redmine_git_hosting was sometimes issuing a working SSH connection to the gitolite user, and sometimes not (acting like there were a need to accept the server's key by having its connection postponed by sshd). So I'll stick to simple Gitolite and maybe Discourse. Anyway, hope this helps.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option 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/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Fits CONTRIBUTING.md.