Migrate NPM packages to @flagsmith organisation

Steps: 

 1. Start publishing the packages (`flagsmith` and `react-native-flagsmith`) to the [@flagsmith organisation](https://www.npmjs.com/org/flagsmith)
 2. Deprecate the existing packages

See additional information on the changes required [here](https://stackoverflow.com/questions/43007471/how-to-migrate-npm-package-to-an-organization-scope). 