Make post delete script also delete context

notifier/database/queries/delete_non_notifiable_posts.sql

@@ -1,3 +1,5 @@
+START TRANSACTION;
+
 -- Purge posts that are no longer considered notifiable
 
 DELETE
@@ -69,4 +71,26 @@ WHERE
         OR thread_sub.sub = 1
         OR post_sub.sub = 1
       )
-  );
+  );
+
+-- Purge context that is no longer relevant to any post
+
+DELETE FROM context_forum_category
+WHERE NOT EXISTS (
+  SELECT NULL FROM notifiable_post WHERE
+    notifiable_post.context_forum_category_id = context_forum_category.category_id
+);
+
+DELETE FROM context_thread
+WHERE NOT EXISTS (
+  SELECT NULL FROM notifiable_post WHERE
+    notifiable_post.context_thread_id = context_thread.thread_id
+);
+
+DELETE FROM context_parent_post
+WHERE NOT EXISTS (
+  SELECT NULL FROM notifiable_post WHERE
+    notifiable_post.context_parent_post_id = context_parent_post.post_id
+);
+
+COMMIT;