Mechanism for displaying summary of m2m relationships in rows on table view

[simonw]

Part of #354 (m2m support)

It would be fantastic if rows that are part of a m2m relationship could display it in an additional column in the table view.

It might look something like this: https://russian-ira-facebook-ads.datasettes.com/russian-ads-919cbfd/display_ads?_search=black+lives+matter

That example was achieved using a custom SQL query and datasette-json-html - but I'd like this to be a built-in feature instead.

[simonw]

Ideally we would display a limited number of m2m related records with a "..." if there are more than our limit. I could also show a count of the total number of records, but this would have to be agressively time-limited or it could cause extremely poor performance.

This could be implemented as a SQL query for every displayed row, taking advantage of Many Small Queries Are Efficient In SQLite. Provided that SQL runs against an index this should be fast to display even on a table with hundreds of rows.