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. 

**sandbox:** http://studio.polesye.m.sandbox.edx.org/

Studio -> Content -> Pages
Before: 104 requests, 1.6MB transferred, load: about 6.7 - 7.2 s, DOMContentLoaded: about 1.1 s
After: 103 requests, 1.4MB transferred, load: about 6.1-6.4 s, DOMContentLoaded: about 1.1 s

@cahrens , @tymofij please review.
> Not a big deal, but I am still confused why we aren't treating the return value from wait_for_xmodules_to_load the same way that we treat the return value from load_requirejs_modules.
> 
> result = wait_for_xmodules_to_load()

Sorry, fixed.