fix: IndicatorPeriod.get_root_period() not available in migrations

The function was copy-pasted from the model class for the migration
 because apps.get_model() can't make model methods available.

#5145: Feature Request: UI for aggregation tasks

akvo/rsr/migrations/0222_indicatorperiodaggregationjob_root_period.py

@@ -4,11 +4,18 @@
 import django.db.models.deletion
 
 
+def get_root_period(period):
+    root = period
+    while root.parent_period:
+        root = root.parent_period
+    return root
+
+
 def set_root_period(apps, schema_editor):
     IndicatorPeriodAggregationJob = apps.get_model("rsr", "IndicatorPeriodAggregationJob")
     jobs = []
     for job in IndicatorPeriodAggregationJob.objects.all():
-        job.root_period = job.period.get_root_period()
+        job.root_period = get_root_period(job.period)
         jobs.append(job)
     IndicatorPeriodAggregationJob.objects.bulk_update(jobs, ["root_period"])