Lint and test the codebase with gitlab CI
Adding a gitlab-ci job for linting the code at every commit and running automated tests using gitlab-ci would be a great improvement for developer productivity and would ensure constant quality assurance. At the moment the .gitlab-ci.yml file is only used to generate the .iso images. Setting up runners would probably be necessary for cross-platform testing. It could use clippy for this purpose relating this to #351 (closed) or simply run rustfmt as suggested in the docs' style page. I've created this issue in response to that page.