Spreadsheet import

If we could detect what columns map to what fields (or provide an interface that lets users specify what field each column should map to post-upload), we could probably allow uploading spreadsheets to import content. Might be a good way to help users who already have lots of content in another format but want to try Notebook.

The issue names no files, tests, or entry points. Start by clarifying whether automatic column detection or a post-upload mapping interface is required, then define the supported spreadsheet formats and import behavior. Done means users can import existing spreadsheet content into Notebook with each column mapped to the intended field.

Written by the indexing model from the issue text.