Allow users to create buildables from diffs

Summary:
Now that diffs have PHIDs we can create buildables for them.

This also adds buildable.diff in the variables list so the diff ID is available, and it also fixes the Cancel button on "Edit Plan" page so it redirects to the right place.

Test Plan: Created a buildable from a diff, ran a build plan against it that had echo ${buildable.diff} and got the right ID. Also tested the "Edit Plan" cancel redirect.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D7546