treat .paper_text as user generated content

We apply some special css to user generated content:

-   `img {max-width: 100%}`
-   smaller headings

`.paper_text` was not interpreted as user generated content resulting in issues like this one: 
![treat-papertext-as-ugc](https://f.cloud.github.com/assets/202576/1666634/f4e41e5a-5c49-11e3-8d91-af4e0d595cd8.png)

This fixes the problem by adding `.paper_text` to the list of user genrerated content elements treating it just as `.proposal .body`.