Allow admins to block access to the whole site for visitors or some roles

[stevejburge]

This is a spin-off from #552

The original request from a user was to put his entire site behind a login screen.

@agapetry's thought:

After reviewing the original request again, I think this would be best implemented by redirect. For each role, there could be an option to either redirect to the home page or to 404 on any URL.

This would be a natural fit and simple code logic for the Edit Role screen in Capabilities Pro. It could be either a new "Block Access" vertical tab or a new section on the "Redirects" tab.