Support password protection

[joelambert]

What does this implement/fix? Explain your changes.

This adds support for password protected pages. Following the Timber guidelines this implements a middleware for pages/posts served via WordPress controllers that will render a special twig template if post_password_required() returns true.

The default template is single-password.twig but can be modified in the theme using the filter lumberjack/password_protect_template.

To ensure this isn't a breaking change if Timber::compile() returns false the middleware gracefully fallsback to the current behaviour, which is to ignore the password protect preference.