Proper refreshing of referenced entities

[JonasWanke]

Maybe we should automatically force-fetch items if you land on the screen? (Like, force-fetch Courses on the CoursesScreen, force-fetch assignments on the AssignmentsScreen, force-fetch submissions if visiting the Submissions tab etc.) Also, if an Entity is force-fetched should we go to all referenced entities and force-fetch them too? Seems like an expected behavior to me: If reloading the assignments screen and a course got renamed or a different color, the CourseChip should also update (but then again, not the teachers which are part of the course, because they are irrelevant 🤔). ^{@marcelgarus at https://github.com/schul-cloud/schulcloud-flutter/pull/223#issuecomment-608067350}