This: 1. Deletes XModuleDescriptorToXBlockMixin 2. Deletes `metadata_traslations` and `_translate()` from `XmlParserMixin`, and all related implementations. 3. Removes translations of deprecated OLX attributes during import/export: <old_attr> : <new_attr> - 'slug' : 'url_name' - 'name' : 'display_name' - 'id' : 'discussion_id' (Discussion XBlock) - 'for' : 'discussion_target' (Discussion XBlock) - 'attempts' : 'max_attempts' (CAPA)
Fix for MA-187 where a value error was thrown when a course has a video directly under a section. This PR handles the situations where a chapter or section can be none, and then returns the appropriate url if available.
Also fixes STUD-1351
