R1:fb9d036e574f

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.