Welcome screen : responsive dimensions

Sur un petit écran:

![image](https://user-images.githubusercontent.com/18699331/63638151-d00ce700-c684-11e9-9039-9095b0254615.png)

![image](https://user-images.githubusercontent.com/18699331/63638157-d9964f00-c684-11e9-9690-4c30fdd900a5.png)

![image](https://user-images.githubusercontent.com/18699331/63638167-e3b84d80-c684-11e9-8d48-a98c524af8c1.png)

