Summary:
- Use edges to store "X depends on Y" information in Maniphest.
- Show both "Depends On" and "Dependent Tasks".
- Migrate all the old edges.
Test Plan:
- Added some relationships, migrated, verified they were preserved.
- Added some new valid relationships, verified tasks got updated with sensible transactions and sent reasonable emails.
- Tried to add a cycle, got an ugly but effective error.
Reviewers: vrana, btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T1162
Differential Revision: https://secure.phabricator.com/D3006
