Thanks kmmbvnr for the report.

Refs #23685.

This is the result of Christopher Medrela's 2013 Summer of Code project.

Thanks also to Preston Holmes, Tim Graham, Anssi Kääriäinen, Florian
Apolloner, and Alex Gaynor for review notes along the way.

Also: Fixes #8579, fixes #3055, fixes #19844.

De-aliased pre/post_syncdb to pre/post_migrate to increase
backwards-compatibility.

By avoiding to run syncdb with the full set of test models.

Thanks Anssi for the idea.

The recent improvements should make shared_models obsolete.

This reverts commit 1059da8d, reversing
changes made to 4fa7f3cd.

Only make assertions that actually test the functionality of the
change and not unrelated pieces.