Use Edges to store dependencies between revisions in Differential

Summary: See D3006. Move this data to the edge store.

Test Plan:

• Created dependencies, migrated, verified dependencies were preserved.
• Added new dependencies, they worked.

Reviewers: vrana, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1162

Differential Revision: https://secure.phabricator.com/D3007