Add file support in the gradebook feedback

This need to be tested in a Moodle 3.6 site, with the fix for MDL-63820 applied (currently in integration). Create an assignment activity with comments feedback enabled. As a user, send a submission. As a teacher, grade the submission and add some feedback with an embedded image. Access the app as the student. Go to the feedback of the assignment. Check that the feedback with the embedded image is displayed. Access the app as the teacher. Go to the assignment and go to the submission of the student. Check that the feedback with the embedded image is displayed. Edit the feedback. Check that the embedded image is displayed in the rich text editor. Add some text and close the editor. Check that the updated feedback is displayed, including embedded image.