Summary: Fixes T4931. Each new credential should come with the ability to lock the credential permanently, so that no one can ever edit again. Each existing credential must allow user to lock existing credential.
Test Plan: Create new credential, verify that you can lock it before saving it. Open existing unlocked credential, verify that option to lock it exists. Once credential is locked, the option to reveal it should be disabled, and editing the credential won't allow username/password updates.
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: epriestley, Korvin
Maniphest Tasks: T4931
Differential Revision: https://secure.phabricator.com/D8947
