Fixes #9118. Permission policies can now be initialised by passing a model string rather than a class; wagtail.admin.widgets.chooser.BaseChooser had this capability already. Between these, we can adjust wagtail.documents.views.chooser so that no models need to be imported at module load time. As a result, definitions that depend on this module (such as DocumentChooserBlock) can now be included in the same models file as a custom document model, without causing a circular import.