Issue #13: Can now store a preference for which attempt to report to the gradebook.

An additional database field was needed to store which attempt's score would be reported.
Issue #13: Grade-items and grades can now be added to the gradebook on adaptivequiz-save.

Still to do:
 * Save user grades on quiz completion.
 * Manage grade visibility.
 * Manage inclusion of grades in the gradebook at all.
 * Test/implement reset support.
 * Add support for locking of grades.
Issue #13: Allowing the gradebook to manage grade visibility.

Visibility isn't being managed in the Adaptive Quiz settings, so we might as
well let the gradebook set visibility. This feature setting was in the initial
import, likely copied from the quiz module even though it wasn't needed.

Still to do:
 * Save user grades on quiz completion.
 * Test/implement reset support.
 * Test/implement support for locking of grades.
Issue #13: Grades are now submitted to the gradebook on quiz completion.

Still to do:
 * Test/implement reset support.
 * Test/implement support for locking of grades.
Issue #13: Gradebook now redirects directly to scoring table if the current user has permission.
Issue #13: Gradebook is now updated when deleting Adaptive Quiz attempts.

Note: testing of grade locking indicates that the gradebook requests refreshed
grades automatically when the grade item is unlocked. No additional work
is needed to support this behavior.

Still to do:
 * Test/implement reset support.
Issue #13: Adaptive Quiz now supports Course Reset.
Issue #13: Adaptive Quiz now deleted grade items when deleting the instance.
Cleaned up unneeded variables/comments.
Now returning proper status messages when resetting the course.
Merge branch 'adpq-13-score-in-gradebook' into MOODLE_23_STABLE

This fixes #13 - Report score into gradebook.