'Parses OLX into XBlock.
This method is overridden here to allow parsing legacy OLX, coming from discussion XModule.
XBlock stores all the associated data, fields and children in a XML element inlined into vertical XML file
XModule stored only minimal data on the element included into vertical XML and used a dedicated "discussion"
folder in OLX to store fields and children. Also, some info was put into "policy.json" file.
If no external data sources are found (file in "discussion" folder), it is exactly equivalent to base method
XBlock.parse_xml. Otherwise this method parses file in "discussion" folder (known as definition_xml), applies
policy.json and updates fields accordingly.'

@classmethod
def parse_xml(cls, node, runtime, keys, id_generator):

    block = super(DiscussionXBlock, cls).parse_xml(node, runtime, keys, id_generator)
    cls._apply_translations_to_node_attributes(block, node)
    cls._apply_metadata_and_policy(block, node, runtime)
    return block
