Add button for creating forum thread draft from event detail

TODO: Allow RIGS to ingest POST requests sent from the forum on new posts in RIG info to link up the forum thread

RE https://forum.nottinghamtec.co.uk/t/rigs-discourse-integration/15592/21

RIGS/migrations/0050_event_forum_url.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.2.18 on 2023-06-26 17:46
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('RIGS', '0049_auto_20230529_1123'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='event',
+            name='forum_url',
+            field=models.URLField(blank=True, null=True),
+        ),
+    ]