fix: threads/comments timestamp not correct

[taimoor-ahmed-1]

This PR fixes the issue when migrating threads/comments from mongodb to mysql timestamps were not being migrated correctly.

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[asadali145]

tested timestamps locally and looks good

[blarghmatey]

Thanks for the fix! Any chance we can get this into a release and have the package bumped in edx-platform? It would also be helpful to get that change backported to Ulmo