Create migration checking for old data requiring Lesrooster to be installed

aleksis/apps/alsijil/migrations/0024_check_new_models.py

+from django.db import migrations, models
+
+from django.apps import apps as global_apps
+
+def check_for_migration(apps, schema_editor):
+    if global_apps.is_installed('aleksis.apps.lesrooster'):
+        return
+
+    ExcuseType = apps.get_model('alsijil', 'ExcuseType')
+    PersonalNote = apps.get_model('alsijil', 'PersonalNote')
+    LessonDocumentation = apps.get_model('alsijil', 'LessonDocumentation')
+
+    model_types = [ExcuseType, PersonalNote, LessonDocumentation]
+
+    for model in model_types:
+        if model.objects.exists():
+            raise RuntimeError("You have legacy data. Please install AlekSIS-App-Lesrooster to migrate them.")
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('alsijil', '0023_add_tardiness_and_rework_constraints'),
+    ]
+
+    operations = [
+        migrations.RunPython(check_for_migration),
+    ]