Set $can_edit for Harbormaster steps
Summary: Sets the $can_edit value correctly (previously it was hardcoded to true).
Test Plan: Went to http://phabricator.local/harbormaster/step/view/1/ and saw "Edit Step" disabled.
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: Korvin
Differential Revision: https://secure.phabricator.com/D14373