Show task duplicates as related objects in Maniphest and migrate old duplicates

Summary: Does the UI work that's part of T12234 and adds migrations for both of the old-style duplicate transactions.

Test Plan:

• Started with a clean DB.
• Checked out really old code that marks tasks as dupes using comments.
• Made a bunch of tasks and closed some as dupes. Made a bunch of additional comments.
• Checked out D10427 and did a storage upgrade.
• Made a bunch more new tasks and dupes.
• Snapshotted DB.
• Ran migration repeatedly until all expected edges showed up in the phabricator_maniphest.edgetable.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T12234

Differential Revision: https://secure.phabricator.com/D18037