[NEW] Empty room background (#412)

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@RocketChat/ReactNative

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Closes #398 

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![aug-09-2018 11-35-32](https://user-images.githubusercontent.com/804994/43906080-cbfadf92-9bc8-11e8-9ac9-44f43d3af023.gif)
![aug-09-2018 11-35-16](https://user-images.githubusercontent.com/804994/43906082-cc19411c-9bc8-11e8-9892-c65c86951a91.gif)
![image](https://user-images.githubusercontent.com/804994/43911366-ad830cd0-9bd5-11e8-8913-6a7e87a2206c.png)