Prevent locked credentials from being made accessible via conduit

Summary:
Via HackerOne. Currently, you can use "Lock Permanently" to lock a credential permanently, but you can still enable Conduit API access to it. This directly contradicts both intent of the setting and its description as presented to the user.

Instead:

• When a credential is locked, revoke Conduit API access.
• Prevent API access from being enabled for locked credentials.
• Prevent API access to locked credentials, period.

Test Plan:

• Created a credential.
• Enabled API access.
• Locked credential.
• Saw API access become disabled.
• Tried to enable API access; was rebuffed.
• Queried credential via API, wasn't granted access.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D15944