[NEW] Onboarding (#407)

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@RocketChat/ReactNative

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Closes #392 

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![aug-07-2018 17-03-50](https://user-images.githubusercontent.com/804994/43799447-f62074dc-9a63-11e8-8aac-bf2c4c5a8a2b.gif)
![aug-07-2018 17-03-35](https://user-images.githubusercontent.com/804994/43799446-f5f84a70-9a63-11e8-8947-265113ae9bf4.gif)
![aug-07-2018 17-03-13](https://user-images.githubusercontent.com/804994/43799445-f5d70ee6-9a63-11e8-94a9-f49c7d69fbba.gif)