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.
> Should you pay attention to the return value from wait_xmodules_to_load()? 

I think no, we just need to know when they are loaded.


> And if result was false from load_requrejs_modules(javascript), is there any point to calling wait_xmodules_to_load()?

Good point. It makes sense do not call `wait_xmodules_to_load`.

> Also, it would be awesome of that complicated JS expression could be shared with lines 238 and following. I'm not sure if that is feasible though (and longterm hope is for Lettuce tests to go away). 

I'll try to do that.

> Speaking of which... do we need similar code over in bok choy land? Since nothing is failing at the moment, I guess we can just wait.

Not now. I'm using `wait_xmodules_to_load` for `TabsEdit`, because I have not found any flag that says that it is initialized. Without this fix, Selenium starts work with the page before initialization of `TabsEdit`.