getting the ACCESS_TOKEN from env

[felix-ht]

Split out from #723

Having users hardcode their token is a bad idea:

1. it is easy to miss while setting it up
2. it is easy to miss while doing a pull request

The new solution uses String.fromEnvironment. If the token is not set an error screen is shown.

To set the token the user passes it with flutter run --dart-define=ACCESS_TOKEN=YOUR_TOKEN. It's also easy to set this variable in IDE's as well.

[shroff]

Great idea @felix-ht. Feel free to add a comment as suggested by @m0nac0 before pushing, though I do think it's fine without it since you show the message in the example app.