Add 'login required' site setting

Based on the conversation in #951 and [meta/4580](http://meta.discourse.org/t/can-discourse-be-configured-to-always-require-login/4580)

This PR adds a 'login required' site setting, for those that want to restrict usage of the forum to logged-in users only :lock:

When activated:

- All routes redirect to `/login` when user is not logged in **except for static pages**
- Login page currently displays a generic "Welcome blah blah blah, please sign in". See `login.en.html.erb` in [`122f9188`](https://github.com/chrishunt/discourse/commit/122f9188c9c2eb9c928930d63404f3e18ddac6ae). I'm happy to add a customization field in admin if people want to customize this blurb (in a separate PR)