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.
Backport of 0385dad073 from master
Fixed MigrationWriter.serialize() to correctly handle migration
operations by utilizing OperationWriter.
Thanks Piotr Maliński for the report.
Backport of e8e4f978dd from master
get_language() can return None when translations are deactivated.
Thanks Nicola Peduzzi for the reporti and Tim Graham for the review.
Backport of 7a0d9b5cda from master.
