Put i18n.js in core_deps instead of in shared

Summary:
live-editor's shared is loaded in webapp, where i18n.js will already exist.
I think this belongs in core_deps, which isn't loaded in webapp, alongside
jquery and friends.

It's interesting to note that i18n.js in live-editor has both the contents of
i18n.js from webapp, and also handlebars helpers defined in
javascript/shared-package/handlebars-extras.js to do with i18n things.

Test Plan: Not sure what the procedure is here

Reviewers: pamela, john

Reviewed By: john

Subscribers: alexrodrigues, #cs

Differential Revision: http://phabricator.khanacademy.org/D14628