3 layer backend architecture, testing suite, and minor improvements

[jenul-ferdinand]

• Setup a three layer backend architecture with an updated file structure, repositories/, services/, controllers/.
• Previous "services" are now providers/. E.g., AiOverviewProvider.
• Setup module alias importing e.g., "@models/unit" instead of "../../../../models/unit". This needed some tweaks so that intellisense works see backend/jsconfig.json and backend/package.json.
• Old endpoints are V1 and new endpoints are V2.
• Testing suite can be found in tests/, using Jest with sample data from the test db as fixtures, mongodb-memory-server is used to test the service layer.