removing unused redirect_from lessons

[ZoeLeBlanc]

This PR removes unused redirect_from field from lessons and preserves redirects for actual redirects. Also closes #2093

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[walshbr]

Great! @ZoeLeBlanc I think that seems to have done it, but @jenniferisasi should take a look and approve as well, since she was the one who found the original errors in it.

Also you should probably note that this closes #2093, right?