linkify citations

[JoshData]

As @DanielSchuman wrote in #3,

it also may be useful to have the citations to the US code link to the US Code (using the pre-built parser) or the statutes (which are all available online at legislink.)

Luckily we have lots of code to do that. Adding 18f/linkify-citations to the page template (without changing the report HTML) is probably the easiest solution. (I've long wanted to submit a PR to https://github.com/18F/linkify-citations/blob/master/src/linkify-citations.js#L25 to use the new linking information in https://github.com/unitedstates/citation.)