Merge Mapper candidates with the same code into a single row

[paynejd]

In multi-algorithm retrieval, it is possible that the same code is returned by more than one algorithm. With bridge matching, it is possible that the same code is returned by more than one bridge candidate, possibly with different map types. For certain views of the Mapper candidates tab, like grouping by Match Quality, it is more intuitive to merge candidates that share the same code into a single row.

Follow up ticket to #2299

This ticket defines requirements and UI designs for the Candidates Tab to support this (@paynejd to add)