fix: Article slug auto saves (#5524)

- Auto save article slug

db/migrate/20220930025317_add_unique_index_to_slug.rb

@@ -0,0 +1,6 @@
+class AddUniqueIndexToSlug < ActiveRecord::Migration[6.1]
+  def change
+    remove_index :articles, :slug
+    add_index :articles, :slug, unique: true
+  end
+end