Use the 'real' path to images now that we no longer compress third_party.zip.

Summary:
We had to go through hoops with the _datatable_static directory.  No
longer.  Now we just need to go through hoops in app.yaml to upload
the necessary files. :-)  (That'll happen in a webapp commit.)

Test Plan: Fingers crossed.

Reviewers: mattfaus

Reviewed By: mattfaus

Differential Revision: http://phabricator.khanacademy.org/D6024