Add dependencies to releeph

Summary: Add the 'Depends On' field to releeph requests. This will help the release engineers to be aware of the dependencies and make sure pick them altogether.

Test Plan: Check sandbox. This field shows up when a revision has some dependencies.

Reviewers: JoelB, lifeihuang, epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7946