Storybook: add viewports addon

Since we need to develop components for different devices we should have testing available in storybook.

Using @storybook/addon-viewport from storybook should help with that: https://github.com/storybookjs/storybook/tree/master/addons/viewport#storybook-viewport-addon

TODO:

• decide which viewports we should focus on
• set breakpoints for each device (or groups of devices) e.g. mobile, pads, desktop-medium, desktop-big
• improve component-stories which should render in different sizes by adding multiple breakpoint views