Factor in language when counting distinct content items

[jamalex]

Summary

Description of the change(s) you made

Ideally, we want the "same" piece of content across multiple languages to have distinct content_ids, so that progress isn't shared etc. We already knew this wasn't the case for Khan Academy channels, for legacy reasons, but it turns out there are other channels in the same boat. This PR calculates uniqueness when counting items in the library as a combination of content_id and language_id.

This bumps the count up from 147K to almost 190K distinct content items in the public library.

Manual verification steps performed

1. Tested out management command and ensured the numbers all made sense

Contributor's Checklist

Testing:

• Contributor has fully tested the PR manually

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Reviewer's Checklist

This section is for reviewers to fill out.

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[jamalex]

(this isn't urgent, as I was able to get the counts manually)