Remove transactions to improve performance

This might be a little bit controversial :smile: - here is the reasoning:

- This was the only way we could scale our forums to the traffic it supports today
- We've been running this way for ~2 years and didn't have any problems because of it (that we know of :smile:)
- [TransactionMiddleware is deprecated since Django 1.6](https://docs.djangoproject.com/en/dev/topics/db/transactions/#transaction-middleware)