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

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1162

Differential Revision: https://secure.phabricator.com/D3006