register cms twig environment

[mjauvin]

One can now use App::get('twig.environment') to get CMS twig environment.

[mjauvin]

Twig::parse() can now render CMS Twig Extensions as well as system ones.

[LukeTowers]

@mjauvin see #3575, that's the one that should be polished up and tested. Note that we don't want the CMS Twig environment to pollute the global one, I believe that causes issue with the Mail system's Twig environment, especially since it also has "partial" tags.

[mjauvin]

I find #3575 too complex and I don't really see the point. My simple fix is enough. I agree we should use cms.twig.environment, though.