Show referring tables and rows when the referring foreign key is compound

[fgregg]

sqlite foreign keys can be compound, but that is not as well supported by datasette as single column foreign keys.

in particular,

1. in a table view, there is not a link from the row to the referenced row if the foreign key is compound
2. in a row view, there is no listing of tables and rows that refer to the focal row if those referencing foreign keys are compound.

Both of these issues are discussed in #1099.

This PR only fixes the second one, because it's not clear what the right UX is for the first issue.

Some things that might not be desirable about this approach.

1. it changes the external API, by changing column => columns and other_column => other_columns (see inline comment for more discussion.
2. There are various places where the plural foreign keys have to be checked for length and discarded or transformed to singular.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.12%. Comparing base (e4ebef0) to head (1e5b42f).
⚠️ Report is 613 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2003      +/-   ##
==========================================
+ Coverage   92.11%   92.12%   +0.01%     
==========================================
  Files          38       38              
  Lines        5555     5565      +10     
==========================================
+ Hits         5117     5127      +10     
  Misses        438      438              

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[fgregg]

because we don't have a strong idea of how to display compound foreign keys in a table view, we have to do this operation in a few places.

[fgregg]

this is how the API changes.

[fgregg]

this is also another API change. this got introduced so we could have compound keys look good in the row template, but we could do that through a custom filter instead.

[fgregg]

@simonw, let me know what you think about this approach!

[fgregg]

see this related discussion to a change in API in sqlite-utils simonw/sqlite-utils#203 (comment)