Save state of pivot tables sections (expanded or collapsed)

**Is your feature request related to a problem? Please describe.**
- Users can collapse and expand sections, but the state is not saved

**Describe the solution you'd like**
- Save the expanded or collapsed state of pivot table sections in `visualization_settings`

**Describe alternatives you've considered**
- users can manually collapse by clicking top header collapse all button

**How important is this feature to you?**
- saves a user click and may align better with user expectations (i.e. saving saves something as it looks upon saving)

Start by tracing how pivot table sections are rendered and how `visualization_settings` is read and written. The work is done when saving preserves each section's expanded or collapsed state and reopening the visualization restores it.

Written by the indexing model from the issue text.