nixos/apache-httpd/wordpress: copy plugins and themes instead of symlinking

Symlinking works for most plugins and themes, but Avada, for instance, fails to
understand the symlink, causing its file path stripping to fail. This results in
requests that look like:

https://example.com/wp-content//nix/store/...plugin/path/some-file.js

Since hard linking directories is not allowed, copying is the next best thing.