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