Add the pointer tag support for the extracted XBlocks

[ttqureshi]

In this story, we will add support for the pointer tag (OLX) format during course import and export for all extracted XBlocks. To accomplish this, we will have the XBlocks inherit from LegacyXmlMixin and implement any additional functionality needed. We will also remove the existing pointer-tag parsing logic from the XBlocks.

The pointer tag support for the following XBlocks need to be added in this story as they have been extracted out:

• Annotatable XBlock
• LTI XBlock
• Poll Question XBlock
• WordCloud XBlock

The pointer tag support for HTML XBlock has been added in the prior story: #435
Since Video, Problem and Discussion XBlocks' extraction is a WIP, therefore we'll be adding the support in their respective extraction PRs.