Integrate groups better with divisions

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

...n/java/nl/tudelft/submit/service/StudentGroupService.java

src/main/resources/messages.properties

src/main/resources/static/css/main.css

src/main/resources/templates/edition/staff.html

src/main/resources/templates/module/groups.html

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