perf: add index to is_blacklisted and created field for performance improvements of xblocks api

[AfaqShuaib09]

PROD-4136

This PR focuses on the performance improvements for xblocks api for skill fetching by adding db_indexes on the fields pointed out by the explain query.

Merge checklist:

• Any new requirements are in the right place (do not manually modify the requirements/*.txt files)
  • make upgrade && make requirements have been run to regenerate requirements
• ./manage.py makemigrations has been run
  • Checkout the Database Migration Confluence page for helpful tips on creating migrations.
  • Note: This must be run if you modified any models.
    • It may or may not make a migration depending on exactly what you modified, but it should still be run.
• Version bumped
• Changelog record added

Post merge:

• Tag pushed and a new version released
  • Note: Assets will be added automatically. You just need to provide a tag (should match your version number) and title and description.
• After versioned build finishes in GitHub Actions, verify version has been pushed to PyPI
  • Each step in the release build has a condition flag that checks if the rest of the steps are done and if so will deploy to PyPi.
    (so basically once your build finishes, after maybe a minute you should see the new version in PyPi automatically (on refresh))
• PR created in course-discovery to upgrade dependencies (including taxonomy-connector)
  • This must be done after the version is visible in PyPi as make upgrade in course-discovery will look for the latest version in PyPi.

[AfaqShuaib09]

This permission class is already present in TaxonomyAPIViewSetMixin, from which it is inherited, so I believe we can remove it from here