bugfixes from 2024.Q1 release

[shapiromatron]

Fix assorted errors encountered by users with our 2024.Q1 release.

1. Fix regression in data pivot visualizations, where OR filters accidentally filtered all content out except one row (we had a Map, but the key for all items was undefined so there was only one item in the map); regression in data pivot - calculated columns #991
2. Fix an N+1 query error on the reference search page; regression in Render tag UDFs in the Reference Tagging view #913
3. Fix an error when converting a literature reference to a study, introduced by the UDF form integration; regression in Add UDF to study and endpoint forms #906
4. Add an additional check for data pivot file uploads to check the extension