MDL-36712: Assignment - Do not always send feedback available emails when grade is created

The entry in the assign_grades table contains grades feedback and other settings such as locked.
Only send the student an email about new feedback if it is actually grades or feedback that
has been added. This is done by using another state for the mailed flag (2) which means "unset".

Conflicts:

	mod/assign/feedback/file/importziplib.php
	mod/assign/feedback/file/locallib.php
	mod/assign/feedback/offline/locallib.php
	mod/assign/locallib.php