Mark render-to-str deprecated, show how to do it yourself

[holyjak]

Use proper require of react-dom/server and invoke its renderToString, instead of accessing the global js/ReactDOMServer, which might not be available.

I had troubles making sure that js/ReactDOMServer and thus couldn't render to a string. With this change, it works fine.

BEWARE: I don't know whether the use of js/ReactDOMServer is a left-over from old times, or whether it is intentional, e.g. to make sure that we do not require react-dom/server if not needed. So I don't know whether this change is desirable or not.

[awkay]

I think I had some notion that I wanted to make this optional...to avoid code bloat. shadow will make that an extern, and will include it in every build (even if you don't use it. no dead code elimination on js libs). Doing the way I did it means you won't get react dom server in your advanced builds unless you require it elsewhere.

so, while what you have is cleaner, I'd rather see a deprecated tag on the function with a recommendation that you just use require and directly reference it yourself.

[holyjak]

Done!