Change the plotter to use a TextListEditor

Summary: You get a TextListEditor, and you, and you...

Test Plan:
 * Load x90fb24c1d7c42fdd?version=edit locally and
 * verify that it has 5 categories
 * edit said categories

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6420