OSIS-1689 Optimized number queries
OSIS-1689 Optimized number queries

	modified:   base/models/education_group_year.py

uclouvain/osis_louvain,uclouvain/OSIS-Louvain,uclouvain/OSIS-Louvain,uclouvain/osis_louvain,uclouvain/osis,uclouvain/osis,uclouvain/osis_louvain,uclouvain/osis,uclouvain/OSIS-Louvain,uclouvain/osis