add users controller and intermediate username step

app/controllers/sessions_controller.rb

@@ -1,8 +1,14 @@
 class SessionsController < ApplicationController
   def create
     @user = User.from_omniauth(request.env['omniauth.auth'])
-    session[:user_id] = @user.id
-    redirect_to controller: 'landing', action: 'room', name: @user.username
+    if @user.persisted?
+      session[:user_id] = @user.id
+      redirect_to controller: 'landing', action: 'room', name: @user.username
+    else
+      @user.save!
+      session[:user_id] = @user.id
+      redirect_to controller: 'users', action: 'edit', id: @user.id
+    end
   rescue => e
     logger.error "Error authenticating via omniauth: #{e}"
     redirect_to root_path