Fix invalid redirect when issuing actions on buildables

Summary: Caught this with the new redirect validation logic.  The `$return_uri` was being set as just `B123` which is not valid.  Prefixing it with `/` (like is done in `HarbormasterBuildActionController` already) gives the correct result of reloading the buildable's page.

Test Plan: Restarted all builds on a buildable, saw the page reload correctly.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10320