Check bad dates and redirect, instead of 404ing, as necessary and appropriate. 
Adds query, conditional, and canonical Unit Tests.

Props kovshenin, SergeyBiryukov, DrewAPicture.
Fixes #10935.


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Inline documentation for hooks in xmlrpc.php.

The old link for RSD is dead; update to the new one.

props bftrick.
see #25229.

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Document comment_edit_redirect.

props dustyf.
see #25229.

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