R1:9196a6bd9f3c

Use Edges to store dependencies between tasks in Maniphest

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…