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 tasks. Created valid
and invalid dependencies between revisions.
Reviewed By: tuomaspelkonen
Reviewers: davidreuss, jungejason, tuomaspelkonen, aran
Commenters: codeblock
CC: aran, codeblock, tuomaspelkonen, epriestley
Differential Revision: 595