Move sign-out route into authentication

Signing out is realted to authentication so we should move the route
into the related file.
Add /sign-in route for development

This work should allow use to develop Performance Platform without using
signon. If we are in development (have the DEBUG config set) then it
will expose a new route that will setup the session using fake user
information stored in the config file.
Merge pull request #47 from alphagov/add-development-signin

Add development signin