React Native got a new release out last week! We should upgrade.

Release notes: https://github.com/facebook/react-native/releases/tag/v0.56.0

One concrete benefit we'll get is this should fix #2787 (crash on typing an emoji).

The release also includes my fix for the RN issue underlying #2589 (compose box gets very slow), though we've already resolved that issue for ourselves by switching to an uncontrolled TextInput on Android.