Parse "Depends on" in freeform Differential fields

Summary:
I considered also parsing "Depends On:" field in the commit message but it's more involved and I also prefer having this information in the summary where it's more visible.
I also didn't want the field displayed by default so user would have to write "Depends On:" anyway.

Test Plan: Used "Depends on D1" in summary, saw it in dependencies.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5300