Add Tests and JaCoCo Code coverage

[madshermansen]

Lets try to get 80% code coverage here, write tests for the current code

• Add unit tests for service layer (aim for 80%+ coverage) #67

• Add integration tests for API endpoints #68

• Implement test fixtures and factories #69

• Add coverage tools (JaCoCo already in gradle but needs configuration) #70

This makes it so we can change code without being worried about core functionalities being ruined