R1:f49e35deaf08

Basic task dependencies for Maniphest

Summary:
This allows you to edit dependencies. It is a better patch than it used to be.
It depends on D725.

- If you create a cycle, it just throws an exception and aborts the workflow.
It should not do this.
- Tasks which depend on the current task aren't shown in the UI. Need to add a
new table for this.
- Transaction text says "attached Task" but should probably say "added a
dependency on task".

Test Plan: Created valid and invalid dependencies between…