Upgrade "masked" config to "hidden"

Summary:
Ref T7185. We currently have "locked", "masked", and "hidden" config.

However, "masked" does not really do anything. It was intended to mask values in DarkConsole, but Config got built out instead and "hidden" is strictly better in modern usage and protects against compromised administrator accounts. "hidden" implies "locked", so it's now strictly more powerful than just locked.

Remove "masked" and upgrade all "masked" config to "hidden". In particular, this hides some API keys and secret keys much more aggressively in Config, which is desirable.

Test Plan: Browsed things like S3 API keys in config and could no longer see them.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7185

Differential Revision: https://secure.phabricator.com/D11763