Add some syncdb deprecation stuff into the deprecation doc
Fixed #22331 -- Made MigrationAutodetector ignore unmanaged models.

This commit reverts 69d4b1c and tackle the issue from a different angle.
Models remain present in the project state, but are now ignored by the
autodetector.
Fixed field deconstruction tests failures introduced by 1506c71a95.

refs #12030.
Fixed the PostGIS circular imports caused by 1506c71a95.

refs #12030.