Relates to #526

This is a minimal set of changes needed for having query CSVs attempt to download all the rows.

What's good about it is the minimalism.

What's bad about it:

1. We are abusing the _size argument to indicate we don't want truncation, which isn't the most obvious thing. Additionally, there are various checks that make sure the "_size" URL parameter is a positive integer, which we are relying on to prevent overloading.
2. The default CSV on a table page will use the max_returned_rows argument. Changing this could be a breaking change, since that's currently a place that has some facilities for pagination. Additionally, i think there's a limit under the hood somewhere which if we removed could lead to sql timeouts
3. There are similar reasons for leaving the current streaming method alone, as the current methods could allow for downloading very large files that could have a sql timeout if we tried to get them in one go.

📚 Documentation preview 📚: https://datasette--1820.org.readthedocs.build/en/1820/