Enforce code style

[SirYwell]

While the overall code quality of LandLord is pretty good, it should be a long term target to keep it that way. To achieve that, we should settle on a common code style that is enforced before a pull request can get merged.

As a tool, I would propose spotless. It is more modern than checkstyle and allows to automatically fix issues like removing unused imports.

The specific rules would need to be discussed.