Add config to allow non-spring-member sign-in

While I'm exploring sagan app, I needed to work around the admin access since I'm not a spring team member.
I just thought it would be nice to have a configuration that can toggle non spring member sign-in, especially for outside developers who want to develop sagan.  
This is good for local dev.
Since sagan is a web site for spring, it may not make sense to have this feature though...