Stanford XBlock License Cleanup

[e0d]

There was a lack of documentation about collection of Stanford XBlocks that were added to the edx GitHub organization and migrated to the openedx GitHub organization.

We have confirmed with Stanford that it was their intention to contribute those XBlocks to the project.

The XBlocks in questions are:

• xblock-sql-grader
• Xblock-image-modal
• Xblock-free-text-response
• xblock-submit-and-compare
• xblock-qualtrics-survey

Two require some additional steps.

• xblock-sql-grader needs a License file added to it
  feat: add missing license file xblock-sql-grader#63
• xblock-free-text-response is a fork of a fork for reasons that are lost to history, the connection should be severed.

We should decided whether it makes sense to preserve the forks back to the original Stanford code.