Clean up and optimization of the Video XBlock in the xblocks-contrib

[farhan]

1. Sync-up the original code in the first commit (Rebase the original code with the edx-platform )
2. Cleaned up the legacy Pointer Tag implementation and use LegacyXmlMixin
3. Make Extracted Video XBlock to use VideoConfigService
4. Use RelativeTime from xblocks.core
5. Do other cleaning work to push the extraction near finishing

[farhan]

PR has been cleaned up for lower/lowest code change set.
openedx/xblocks-contrib@f409005

Testing on edx-platform is pending have created the sand-box in the following PR:
openedx/edx-platform#37851

[farhan]

I have created following issue story while doing testing on the created sanbox.

#479