R1:eccbbce9a282

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…