While the table fields are loading we show a skeleton status:
https://www.figma.com/file/4sXj42XyicBsdQkYIugHaV/Compass-Import%2FExport?node-id=923%3A45170&t=FoEPTv8iHfB487DG-1 

Once the fields have completed loading we show them with checkboxes, similar to the existing export. We also show a new field input button and input field: https://www.figma.com/file/4sXj42XyicBsdQkYIugHaV/Compass-Import%2FExport?node-id=920%3A44732&t=FoEPTv8iHfB487DG-1 

This work involves using the `gatherFields()` function from the store to get the field names to show.

Designs: https://www.figma.com/file/4sXj42XyicBsdQkYIugHaV/Compass-Import%2FExport?node-id=923%3A45170&t=FoEPTv8iHfB487DG-1