categorization: make css more robust

Summary:
make the last css fix more robust
by using a longer classname.

Test Plan:
 * subperseus
 * make a categorization in devappserver
 * make sure the item and category fit next to each
   other in the editor

Reviewers: brianmerlob

Reviewed By: brianmerlob

Subscribers: alpert

Differential Revision: http://phabricator.khanacademy.org/D7420