Don't disable the "Show Public Key" action for locked credentials

Summary: Fixes T5104. It's still OK to reveal public keys of locked credentials -- the controller has the right logic, this UI just isn't in sync.

Test Plan: Viewed passphrase; saw enabled menu item on locked credential.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5104

Differential Revision: https://secure.phabricator.com/D9186