[Bug 867221] Show hot questions on product pages.

This adds questions labeled hot to the product "Hot Topics" section. I also added an icon to distinguish between documents and questions using the same icons that we used in search. This is behind the waffle flag `hot_questions`.

To test:

1. Enable the flag
2. On any question, add the tag "hot".
3. Check out the product page for the appropriate tag.
4. Add more tags. Note that
  - Only the right product is shown
  - Only 3 are shown.
5. Turn the flag off. Note that the product page reverts to the original behavior.

r?

![Screenshot](https://f.cloud.github.com/assets/305049/765116/55a8b996-e821-11e2-9987-45aeb56265c4.png)