MDL-37635 Assign: Prevent errors when viewing a feedback for an assignment with no gradeitem in gradebook

Unit tests removed from backport to verify that:
Feedback is visible even if there is no grade
The grade is not shown if the gradeitem is hidden unless you have mod/assign:grade capability

Conflicts:

	mod/assign/locallib.php
	mod/assign/renderer.php
	mod/assign/tests/locallib_test.php