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[bot]]

Codecov Report

Base: 92.11% // Head: 92.12% // Increases project coverage by +0.01% :tada:

Coverage data is based on head (1e5b42f) compared to base (e4ebef0). Patch coverage: 100.00% of modified lines in pull request are covered.

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              

Impacted Files	Coverage Δ
datasette/app.py	94.50% <100.00%> (+0.01%)	:arrow_up:
datasette/filters.py	95.73% <100.00%> (+0.04%)	:arrow_up:
datasette/utils/__init__.py	94.89% <100.00%> (+0.02%)	:arrow_up:
datasette/views/row.py	87.82% <100.00%> (ø)
datasette/views/table.py	92.61% <100.00%> (+0.03%)	:arrow_up:

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[fgregg]

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

[fgregg]

see this related discussion to a change in API in sqlite-utils https://github.com/simonw/sqlite-utils/pull/203#issuecomment-753567932