Display intros to interpretations

* Modify the single paragraph chrome view to bring it up to snuff with the rest of the chrome views; largely this means using the same template and building a tree around the requested paragraph
* Add any introductory paragraphs to the interpretations to the TOC
* SectionUrl now respects those introductory paragraphs

(This is the display side of https://github.com/cfpb/regulations-parser/pull/175)

There are some JS issues (TOC doesn't highlight, Nav isn't right), but nothing deadly.