R1:9afbb9b83b5f

Route task merges through new editor

Summary:
Ref T2217. Ship "Merge in Duplicates" through the new editor. The only notable thing here is `setContinueOnMissingFields()`.

The problem this solves is that if you add a custom field and mark it as required, all existing tasks are "invalid" since they don't have a value, and trying to edit them will raise an error like "Some Custom Field is required!". This is fine for normal edits via the UI, since the user can just select/provide a value, but surgical edits…