Redo assignment page

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

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

...ain/java/nl/tudelft/submit/service/SubmissionService.java

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

src/main/resources/templates/assignment/student_view.html

src/main/resources/templates/assignment/teacher_view.html

src/main/resources/templates/assignment/view.html

src/main/resources/templates/submission/dropzone.html

...l/tudelft/submit/controller/AssignmentControllerTest.java

...va/nl/tudelft/submit/service/StudentGroupServiceTest.java