Implicitly export ascendant topics?

[JeremyCaney]

Currently, when a topic is exported, the exported topic graph starts at the topic being exported. Then, when it's imported, it necessitates that all parent topics already exist.

As an alternative, we could implicitly export all ascendant topics and flag them so that they're always imported using the Add import strategy. That way, there is always enough contextual information to import a topic, but it won't interfere with existing topics outside of the export scope.