Switch from the localizely Flutter Intl plugin to the new Flutter internationalization / localization

[stephanegigandet]

The app currently use the Flutter Intl plugin from localizely to manage translations:
https://github.com/localizely/flutter-intl-plugin-sample-app

Flutter 1.22 announce https://medium.com/flutter/announcing-flutter-1-22-44f146009e5f#f2cb has a section "New internationalization and localization support" with a link to a Flutter Internationalization Guide in a google doc: https://docs.google.com/document/d/10e0saTfAv32OZLRmONy866vnaw0I2jwL8zukykpgWBc/edit

The 2 approaches look very similar (using .arb files to automatically generate dart code), but if there is now an official way, we probably should use it. The differences seem to be mostly in the name of the directories for the arb files and the generated code, as well as the name of the function to access the localized strings.