SPIKE: Display a Content from the Web in a WebView

We appear to be having a little challenge getting **Google Auth** to work in **Flutter _without_ Firebase**: https://github.com/dwyl/learn-flutter/issues/4
This is a **_temporary_ obstacle** if we are able to _embed_ the fully functional Phoenix Google Auth into our Flutter App using a WebView. If we are able to display _any_ web page in a Flutter App then we will be able to perform all the Google Authentication steps in a [web view](https://pub.dev/packages/webview_flutter) and _eliminate_ the need for Firebase.

# Todo

+ [x] **Create** a new file in this repository called **`webview-tutorial.md`** and
+ [x] **Capture** all the steps required to display content from the web in a Flutter App
+ [x] **Display** a web page e.g: wikipedia in a WebView in a Flutter App and confirm that it is interactive.
+ [x] _Ideally_ we should show how to display the WebView _both_
+ [x] Only one **portion** of the screen
+ [x] ***Full-screen***.

## Useful Links

+ https://medium.com/flutter/the-power-of-webviews-in-flutter-a56234b57df2?
+ Using WebViews in Flutter (_official tutorial_): https://youtu.be/RA-vLF_vnng
+ How to use the WebView Plugin: https://youtu.be/Wo0o0wSkn4k

> **Note**: I am _aware_ that at the time of writing there is an issue displaying the keyboard for input in a WebView https://github.com/flutter/flutter/issues/19718 that may affect our ability to seamlessly use the Elixir Auth, but I trust that it will be resolved soon. Please proceed with this SPIKE and if we determine that it is unworkable to perform Auth in a WebView we will consider alternative options.

@miguelmartins17 in case you are wondering why this issue has the word "SPIKE" in the title,
see: https://en.wikipedia.org/wiki/Spike_(software_development) 😉