Add 'login required' site setting
Redirect all controllers to login if required

We want to skip the filter for sessions controller so that we can login
and we want to skip the filter for static pages because those should be
visible to visitors.
Redirect to root after login if no path provided

If we do not do this, then people that login from /login will just be
redirected back to the login page. We'd rather have them see the root
path.
Show login modal on header buttons if required
Redirect Ember routes to login if login required
Merge pull request #957 from chrishunt/chrishunt/lock-down-the-base

Add 'login required' site setting