R1:1ffa16aa6b38

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

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