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 to specific fields should just ignore these errors. Add the ability to ignore these errors and use it on all the field-speific editors.

Test Plan: Merged duplicates, including "invalid" duplicates with missing fields.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2217

Differential Revision: https://secure.phabricator.com/D7084