This also prevents state modifications from corrupting previous states.
Previously, when a model defining a relation was unregistered first,
clearing the cache would cause its related models' _meta to be cleared
and would result in the old models losing track of their relations.

This reflects the actual behavior of the method.

Fixed MigrationWriter.serialize() to correctly handle migration
operations by utilizing OperationWriter.

Thanks Piotr Maliński for the report.

Changed OperationWriter to support multi-line serialized values with
correct indentation.

The current board has been re-elected.

get_language() can return None when translations are deactivated.
Thanks Nicola Peduzzi for the reporti and Tim Graham for the review.

Thanks Domas Lapinskas for the report and Tim Graham for the
review.

Thanks to gfairchild for the report and Claude for the review.

Thanks to knbk for the report.

Forwardport of e7366ab1 from stable/1.8.x

Fixes a regression introduced in 53c2cf1e.

Forward port of 5483c66f from stable/1.8.x

Forward port of d1b206da from stable/1.8.x

Forward port of 05a781a4 from stable/1.8.x