Add basic validation of item prerequisites

Currently throws the worlds most unhelpful error message...

training/migrations/0005_auto_20220223_1535.py

@@ -0,0 +1,22 @@
+# Generated by Django 3.2.12 on 2022-02-23 15:35
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('training', '0004_rename_name_trainingitem_description'),
+    ]
+
+    operations = [
+        migrations.AlterModelOptions(
+            name='trainingcategory',
+            options={'ordering': ['reference_number'], 'verbose_name_plural': 'Training Categories'},
+        ),
+        migrations.AddField(
+            model_name='trainingitem',
+            name='prerequisites',
+            field=models.ManyToManyField(blank=True, to='training.TrainingItem'),
+        ),
+    ]