Fix some bugs with too early imports

biscuit/apps/chronos/migrations/0012_merge_20191118_1939.py

+# Generated by Django 2.2.6 on 2019-11-18 18:39
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ('timetable', '0011_merge_20191113_1655'),
+        ('timetable', '0010_auto_20190901_1040'),
+    ]
+
+    operations = [
+    ]