Fix migrations for validity range

• wrong base
• wrong unique index

aleksis/apps/chronos/migrations/0002_school_term_validity.py

@@ -42,7 +42,7 @@ def migrate_lesson(apps, schema_editor):
 
 class Migration(migrations.Migration):
     dependencies = [
-        ("core", "0003_auto_20200627_1600"),
+        ("core", "0002_school_term"),
         ("sites", "0002_alter_domain_unique"),
         ("chronos", "0001_initial"),
     ]