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.

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