'Import the given xblock into the current branch setting: import completely overwrites any
existing block of the same id.
In ModuleStoreDraftAndPublished, importing a published block ensures that access from the draft
will get a block (either the one imported or a preexisting one). See xml_importer'

@abstractmethod
def import_xblock(self, user_id, course_key, block_type, block_id, fields=None, runtime=None, **kwargs):

    raise NotImplementedError
