Externalize BASE_URL as environment variable

[joernb]

Currently, the BASE_URL is hardcoded:

private static final String BASE_URL = "https://api.ito-app.org";

By externalizing it as an environment variable, it will become possible to configure the BASE_URL during the build process and have different backends (e.g. staging vs production backend).

Possible solution: https://github.com/luggit/react-native-config