ensure that any password protected pages at set to have no-cache, private

[jkeasley]

add functionality to detect if a page/post is password protected and default to no-cache when this is the case

[jkeasley]

I have added a check at the same priority as the isLoggedIn level for whether a page has a password set, if it does then it will always be set with the header Cache-Control: no-cache, no-store, private

Additionally the header set when we have a logged in admin user will also be Cache-Control: no-cache, no-store, private

https://github.com/dxw/dxw-cache-control/tree/feature/password-protected-no-cache