Don't deprecate columns too soon.

We still need these columns for both the data migration and as the fall back
that is used during the interim period when the code has been deployed but the
data migration has not completed.
Merge pull request #1756 from alphagov/fix-html-attachments

Don't deprecate columns too soon.
Cleanup deprecated HtmlAttachment attributes

This completes the work to move HtmlAttachment body content to the
GovspeakContent model.