Change internal-webserver uses of api/v1 to api/internal

Summary:
The only place this came up was in our perf charts. I think the exercise-related
URLs may not be completely moved over yet, but they should be pretty soon.

We may at some point want to query for either the old or new name in order to
include public API usage and mobile app usage, but I think just using internal
should be fine for now.

Test Plan: Fingers crossed.

Reviewers: jlfwong, csilvers

Reviewed By: csilvers

Subscribers: csilvers, benkomalo

Differential Revision: http://phabricator.khanacademy.org/D13837