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