Set report's reason to cascade deletion if the reason is deleted

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

board/models.py

@@ -247,7 +247,7 @@ class Report(models.Model):
     # The report that was made by this user
     record = models.ForeignKey("ReportRecord", on_delete=models.CASCADE)
     # Reason for this report
-    reason = models.ForeignKey("ReportReason", on_delete=models.SET_NULL, null=True)
+    reason = models.ForeignKey("ReportReason", on_delete=models.CASCADE)
     # IP address of the reporter
     ip = models.GenericIPAddressField()