Add ancestor information to summary and session log exports

[rtibbles]

Observed behavior

Some users have asked for additional information showing the topic hierarchy for each resource node in the export logs. We had prototyped this previously, but it caused huge slowdowns during the export process.

However, now that the ancestor information is directly annotated onto the content nodes, we can reuse this for quick access to this information.

Expected behavior

The ancestors information should be unpacked and the longest ancestor chain used to determine how many topic headers to include. Then each ancestor for each row should be put into a topic header until the ancestors are exhausted.

The topic headers should be a string in the form "Topic level {n}" starting at 1.

User-facing consequences

This should hopefully allow users who have made deliberate choices in their topic hierarchies to use that information in their data analysis.