Motivation for this change

Update to latest stable.

Determining what exactly is the "latest stable" Opera is a bit non-obvious,
but this the version they offered when I attempted to download it manually.

Interestingly '50' isn't in the version history yet.

But it has 'stable' in the URL ;) and the beta version it gives you is different anyway.

Related: http://blogs.opera.com/desktop/2018/01/opera-50-introduces-anti-bitcoin-mining-tool/

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.

I don't use opera normally, but it launches and I was able to visit pages and play youtube videos, so seems fine :).