Allow facet by primary keys, fixes #985

[MrNaif2018]

Hello! This PR makes it possible to facet by primary keys.
Did I get it right that just removing the condition on UI side is enough? From testing it works fine with primary keys, just as with normal keys.
If so, should I also remove unused data-is-pk?

[codecov]

Codecov Report

Merging #986 into main will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #986   +/-   ##
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  Coverage   84.28%   84.28%           
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  Files          28       28           
  Lines        3850     3850           
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  Hits         3245     3245           
  Misses        605      605           

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[simonw]

Thanks for taking a look! The fix ended up being a little different from this because I still want to disable faceting on regular single primary keys (since faceting by those won't ever produce interesting results) - here's what I used: 5d6bc4c