fix(sql): coursewide attempts grouped by problem_id

[Ian2012]

Description

This PR fixes a couple of issues with the datasets:

• Adds the problem_link and video_link to the related datasets allowing them to be filtered.
• Switch from an SQL expression selecting a column to use Superset dataset columns.
• Fixes the logic for coursewide average per problem.
• Joins the at-risk dataset with the coursewide_avg dataset.

[openedx-webhooks]

Thanks for the pull request, @Ian2012!

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