[bug 901664] Remove old topics for good

- This removes the old topics completely.
- The old topics model is used by a couple of the migrations so to prevent them from breaking I created `models.py` in the migrations folder.
- The hot topics landing page will probably need some tweaking. I have filed a follow up bug related to this. 
- The questions <--> topics M2M table has been renamed back to `question_question_topics`.