Double Submission Bug Fix

Fixing the double submission bug for semester, course and assignments.

Adding three new functions in the functions php file

doesSemesterByYearAndTermExist($link, $year, $term, $student_id)

doesSemesterAndCourseExist($link, $semester_id, $designation,
$student_id)

doesSemesterAndCourseAndAssignmentExist($link, $semester_id,
$course_id, $name, $student_id)