Adding flavors

**Is your feature request related to a problem? Please describe.**
As a developer/maintainer, I would like to be able to have a DEV build of the application along with a PRODUCTION build.
This will also set the starting block for the beta environment.

**Describe the solution you'd like**
Each flavour would have a specific app icon (we could change the background colour of the app icon like Chrome canary is yellow or add a dev/debug banner) and a different package id (for dev/debug: normal package id with .dev OR .debug at the end).

I think this [medium article](https://medium.com/flutter-community/flavoring-flutter-applications-android-ios-ea39d3155346) describes the complete process.

**Acceptation criteria**

- [ ] Documentation
- [ ] App icon for each flavour
- [ ] App name for each flavour
- [ ] Package id for each flavour
- [ ] Test (if needed)