Resolve "two visits with the same dates"

Closes #369 (closed), #373 (closed), #372 (closed), #370 (closed)

smash/web/migrations/0192_auto_20210224_1703.py

+# Generated by Django 3.1.4 on 2021-02-24 17:03
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('web', '0191_auto_20201201_1033'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='customstudysubjectfield',
+            name='default_value',
+            field=models.CharField(blank=True, max_length=256, null=True),
+        ),
+    ]