Add a modname class to body (backport of MDL-75844) to 4.0

Backport the improvement MDL-75844 to 4.0.

While being generally a useful thing, most specifically this fix is requred by the StudentQuiz activity, so we can fix this issue: https://github.com/studentquiz/moodle-mod_studentquiz/pull/421. As you can see there, the automated tests are now passing for master, but not for 400 stable. Since StudentQuiz is fairly widely used component that we support no behalf of the community, we would like to be able to fix this for all 4.x versions, so thank you for considering this backport request.