No UI for potential duplicates

[joeroe]

The show and maybe index views for duplicable record types should have a UI for potential duplicates.

The show view should list all duplicate groups the record is a member of. For users with sufficient permissions, it should give the option of triggering a merge of the whole group, or part of it. The user should be able to control which fields should be taken from which source record (cf. QGIS).

An index view could also be useful for working through duplicates in batches, but that might be redundant to a dedicated duplicate group index.

Exact duplicates should be automatically resolved and so don't need a UI.