Refactored Perseus outer level code to be more reacty

Summary:
split hint editing and rendering into its own file (hint-editor.jsx) in
preparation for aligning hints.
Refactored test.html's html into a jsx EditorPage render method, which
owns both the editor and renderer.
Fixed up test.html and testrender.html script includes

Test Plan:
test that test.html and testrender.html appear the same afterwards
subperseus and see if webapp /devadmin/content/items/new works, given
appropriate modifications to perseus-admin.js, et al

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D4259