Lots of small changes; the result of bughunting & Intellij inspection

.gitlab-ci.yml

CONTRIBUTING.md

README.md

src/main/java/nl/tudelft/submit/DevDatabaseLoader.java

.../java/nl/tudelft/submit/controller/EditionController.java

...in/java/nl/tudelft/submit/controller/GradeController.java

...n/java/nl/tudelft/submit/controller/PersonController.java

.../nl/tudelft/submit/controller/StudentGroupController.java

...va/nl/tudelft/submit/controller/SubmissionController.java

.../java/nl/tudelft/submit/controller/VersionController.java

src/main/java/nl/tudelft/submit/enums/GradableType.java

src/main/java/nl/tudelft/submit/enums/ScriptGradePolicy.java

...ain/java/nl/tudelft/submit/external/MultipartUtility.java

src/main/java/nl/tudelft/submit/model/Version.java

...java/nl/tudelft/submit/repository/FeedbackRepository.java

...a/nl/tudelft/submit/repository/SubmitGroupRepository.java

.../nl/tudelft/submit/repository/UserSettingsRepository.java

.../java/nl/tudelft/submit/repository/VersionRepository.java

...nl/tudelft/submit/repository/script/ScriptRepository.java

...java/nl/tudelft/submit/security/AuthorizationService.java