implement /setup and /admin

app/controllers/setup_controller.rb

@@ -0,0 +1,45 @@
+class SetupController < ApplicationController
+  before_action :check_setup_allowed
+
+  def show
+    @user = User.new(role: :admin)
+  end
+
+  def create
+    @user = User.new(user_params)
+    @user.role = :admin
+    @user.invitation_accepted_at = Time.current
+
+    if @user.save
+      create_installed_marker
+      session[:user_id] = @user.id
+      redirect_to admin_root_path, notice: "Setup complete! Welcome to Sanasto Wiki."
+    else
+      render :show, status: :unprocessable_entity
+    end
+  end
+
+  private
+
+  def check_setup_allowed
+    if setup_completed?
+      redirect_to root_path, alert: "Setup has already been completed."
+    end
+  end
+
+  def setup_completed?
+    File.exist?(installed_marker_path)
+  end
+
+  def installed_marker_path
+    Rails.root.join(".installed")
+  end
+
+  def create_installed_marker
+    FileUtils.touch(installed_marker_path)
+  end
+
+  def user_params
+    params.require(:user).permit(:email, :name, :password, :password_confirmation, :primary_language)
+  end
+end