optimize new avatar images for size, re #5005

public/assets/images/avatars/course/nobody_medium.webp

public/assets/images/avatars/course/nobody_normal.webp

public/assets/images/avatars/course/nobody_small.webp

public/assets/images/avatars/course/studygroup_medium.webp

public/assets/images/avatars/course/studygroup_normal.webp

public/assets/images/avatars/course/studygroup_small.webp

public/assets/images/avatars/institute/nobody_medium.webp

public/assets/images/avatars/institute/nobody_normal.webp

public/assets/images/avatars/institute/nobody_small.webp

public/assets/images/avatars/user/nobody_medium.webp

public/assets/images/avatars/user/nobody_normal.webp

public/assets/images/avatars/user/nobody_small.webp