Add helper for scaling units

[nschoen]

Description: The app will only be available in portrait mode on smartphone devices. Thus we don't have consider responsiveness much. But smartphones vary quite much in their pixel density resulting in huge differences when using hardcoded pixel sizes for fonts and sizes. Thus it is encouraged to use flexbox or percentage values when possible and use some unit scale helper when pixel units are required (e.g. for font sizes or sometimes for images).

Acceptance criteria:

• [ ] Add Helper for scaling units
• [ ] Add description to the README which explains the unit scale helper and build in tools to work with screen sizes and pixel densities, encourages to use flex and percentages and shows examples of devices with different pixel densities

[nschoen]

Relevant resources:

• https://facebook.github.io/react-native/docs/pixelratio
• https://medium.com/@shanerudolfworktive/7-tips-to-develop-react-native-uis-for-all-screen-sizes-7ec5271be25c
• https://developerlife.com/2018/07/21/designing-for-multiple-screen-densities-on-android/
• https://stackoverflow.com/questions/33628677/react-native-responsive-font-size/42971004
• https://medium.com/react-native-training/build-responsive-react-native-views-for-any-device-and-support-orientation-change-1c8beba5bc23
• https://medium.com/building-with-react-native/how-to-develop-responsive-uis-with-react-native-1x03-a448097c9503
• https://github.com/DaniAkash/react-native-responsive-dimensions#readme

[nschoen]

Closed for now, does not seem relevant currently