Migration videoalpha module to one main video module

@wedaly 
@valera-rozuvan 
@peter-fogg 
, please review.

@wedaly, we've removed flag `STUB_VIDEO_FOR_TESTING`, cause we are testing video module rendering (https://github.com/edx/edx-platform/commit/74f3595d545d95ca0cf2413dd3cc5bdbef99556a). Please, confirm: is it ok?