Router for api explorer - imitates anchor links without custom ids on elements

Summary: Router that handles link clicks. Due to the fact that React hijacks ids this is a bit of a hack.

Test Plan: check that clicking on menu links actually moves you to a different place on a page

Reviewers: alpert, joel

Reviewed By: alpert

CC: ben

Maniphest Tasks: T1210

Differential Revision: http://phabricator.khanacademy.org/D3010