Save authorPHID on Passphrase Credentials to support "Credential Author" object policy

Summary:
Fixes T5135. Currently, when you create a credential, we default the policies to your PHID. This means we can't have an application-level configurable default because there's no way to select "the actor's PHID" as a policy.

Start tracking the credential author's PHID and add an object policy for it, so there is such a setting.

Then, add policy defaults.

This mostly unblocks T6787. This obsoletes T6860.

Test Plan:

• Created a credential with "Credential Author" policy.
• Verified I can see/edit it, but other users can not.
• Changed default policies to something else.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5135

Differential Revision: https://secure.phabricator.com/D13385