Make podspec great again.

Summary:
Fixes #11272
Fixes #11572
Fixes #11781

The main changes here are:

* This depends on the latest CocoaPods (1.2.0). It’s currently in RC, but if I’m not mistaken a proper release is expected soon. /cc dantoml
* Adds required header search paths for the jschelpers and cxxreact subspecs.
* Makes the jschelpers and cxxreact headers private to building React Native, not visible to the user’s project.
* It uses the canonical upstream Yoga v1.0.0 podspec: https://github.com/facebook/yoga/blob/master/Yoga.podspec
* Consistent styling.

I have been able to get our app to build again using this https://github.com/artsy/emission/pull/437. The spec has some warnings, but otherwise fully passes lint.

rh389 sjmueller Could you please test with your projects?
Closes https://github.com/facebook/react-native/pull/12089

Differential Revision: D4518605

fbshipit-source-id: ecf86232d8b1af52d139eadd1acc10f5c1d42c29