Mobile Device UI Previews

[bonham000]

This PR:

• Create mobile device previews for the UI Development module challenges.
• This uses the mobile device HTML/CSS provided by https://marvelapp.github.io/devices.css/ and the adapted CSS provided by https://github.com/philipkiely/devices.css which allows us to resize the mobile device previews.
• Adjust styles for the mobile previews so they work on mobile devices as well.
• Fix a vertical scrolling bug on mobile.
• Create some helpers for testing React Native challenges more easily: reactNativeTextExists and reactNativeButtonWithTextExists.

Note:

• This just supports default iOS and Android views for now, however the project mentioned above provides other devices (and device color themes). We could expand this to support more options in the future if we wanted. This PR is meant mainly as an initial implementation.

Demo:

iPhone 8 Plus:

Galaxy Note 8:

[bonham000]

I found this by searching for pure HTML/CSS imitations of mobile devices (I was sure they existed) and it turns out this awesome library exists AND someone else modified it to allow the devices to be resizable.

⭐ ⭐ ⭐ Open Source⭐ ⭐ ⭐