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