Dark mode, iframe and escape all work

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

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

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

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

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

src/main/resources/templates/layout.html

src/main/resources/templates/submission/content-overlay.html

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