Extracting out edx-platform related code from Built-In Video Block to video-config app

[farhan]

As eventually we are going to delete the Built-In Video Block (xmodule.video_block) from edx-platform directory.

This story is a sub-task of Parent story
in which we are extracting out Video Block to xblocks-contrib repo.

Let's make a list of code units needed to move out:

1. Move sharing_sites into video_configuration app
2. Make Built In Video Block to start using this utility class introduced while writing VideoConfigService. Tests will be required to fix.
3. Make Built In Video Block to start using VideoConfigService. Tests will be required to fix.