Upgrade to react-native 0.50

[lachlanjc]

https://github.com/facebook/react-native/releases/tag/v0.50.0

[kusti8]

I don't know if we need to keep on updating. It gets very hard to merge conflicts and if we don't need the extra features, there isn't much reason to do it now. I did it last time and it actually took a while. I had to go through the xcode file manually and edit it.