R1:2a7545a452b0

Convert Maniphest merge operations to modern Relationship code

Summary:
Ref T4788. Fixes T7820. This updates the "Merge Duplicates In" interaction, and adds a "Close as Duplicate" action.

These are the last interactions that were using the old code, so it removes that code.

Merges are now recorded as real edges, so we can show them in the UI later on (originally from T9390, etc).

Also provides more general support for relationships which need EDIT permission, not-undoable relationships like merges…