refactor: ⚡ Username and slug unique indexes, before actions for set link and user

confirm_password_validation around action, unnecessary helpers removed, rubocop-rails suggestions applied

Add username index and unique slug index migrations

app/controllers/application_controller.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
 class ApplicationController < ActionController::Base
-  include SessionsHelper
+  def authenticate
+    @current_user = User.find_by(id: session[:user_id])
+  end
 end