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/models.py

@@ -357,6 +357,8 @@ class Event(models.Model, RevisionMixin):
     auth_request_at = models.DateTimeField(null=True, blank=True)
     auth_request_to = models.EmailField(blank=True, default='')
 
+    forum_url = models.URLField(null=True, blank=True)
+
     @property
     def display_id(self):
         if self.pk: