Fixes #9010. `wagtail.documents.widgets` is no longer reliably imported on startup now that the chooser widget is constructed within wagtail.documents.views.chooser instead, so the telepath adapter wasn't getting register. This meant that DocumentChooserBlocks in StreamField were using the base chooser implementation, which didn't include customisations such as populating the title field from the file upload field. Fix this by making ChooserViewSet responsible for registering the telepath adapter.
Fixes #8989. The previous fix #9004 failed for custom document models because ChooserViewset assigns an internal name for the ChooserBlock class based on the model name, and if this is anything other than Document it won't match the name DocumentChooserBlock that it's exposed under in wagtail.documents.blocks. Fix this by replacing the `block_class` property with a `get_block_class` method that lets us specify the class name. As a bonus, user code that defines chooser blocks no longer has to directly hack the `__module__` attribute.
