Make the pointer tag import/export utitlity functions available to the Extracted XBlocks

[ttqureshi]

This story stems from adding the pointer tag parsing support for the Extracted XBlocks. We have decided to add the necessary utility functions to the xblocks-contrib so that these are available to the extracted xblocks and they are exported in the pointer tag format and during import parse the pointer tags correctly as in the BuiltIn Blocks.

[ttqureshi]

We need to groom it further.

cc: @openedx/axim-aximprovements

[ttqureshi]

As discussed in the grooming session, we'll be adopting the following approach for this issue:

1. House the utility functions at a central location (xblocks-contrib/common/xml_utils.py) so that these are accessible to all the extracted xblocks and
2. One-by-one, for every extracted XBlock add pointer tag supoort for the course export/import functionality (either in seperate tickets* or all-in-one).

*The idea of having seperate tickets for every extracted xblock is that it will be easier in code review as well as the testing.