New setting site_requires_login

This feature isn't quite complete yet, as users can see the "outside" of the topics, and they can see full categories, but at least they can't view the content of the topics now when this setting is enabled.

My use case for this is effectively an alternative to the password-blocked site.

I plan on working on this more this weekend, so feel free to either accept or ignore the pull request in its current state.  I will update this PR when I figure out how to handle hiding topics and categories from the outside as well.