Redirect users to original request after login

This commit does two things:

1. It passes `origin: request.url` into the `github_auth_url()` named
   route, which will preserve that request and return it with the
   OmniAuth hash.
2. The `create` action on the sessions_controller then either redirects
   to the originally requested URL or, if that doesn't exist, to the
   old default `posts_url` named route.

app/controllers/application_controller.rb

@@ -19,7 +19,7 @@ def user_signed_in?
 
   def authenticate_user
     unless user_signed_in?
-      redirect_to login_url, notice: "Please log in first"
+      redirect_to github_auth_url(origin: request.url)
     end
   end
 

app/controllers/sessions_controller.rb

@@ -9,7 +9,7 @@ def create
       author.email = omni['info']['email']
     end
     self.current_user = @author
-    redirect_to posts_url
+    redirect_to request.env['omniauth.origin'] || posts_url
   end
   def login
   end