Fix double loading of cms-modules.js.

**Ticket:** [TNL-272](https://openedx.atlassian.net/browse/TNL-272)
**Acceptance criteria:**
 * The module cms-modules.XXXXX.js should be loaded only once on the unit edit page in Studio.
 * Performance improvement should be measured before and after this story is completed. (**Needs to be done.**)

**sandbox:** http://studio.polesye.m.sandbox.edx.org/

@cahrens , @tymofij please review.