feat: APIs for Articles (#4777)

Fixes: #4802

db/migrate/20220527080906_add_reference_for_author_id.rb

@@ -0,0 +1,6 @@
+class AddReferenceForAuthorId < ActiveRecord::Migration[6.1]
+  def change
+    remove_column :articles, :author_id, :integer
+    add_reference :articles, :author, foreign_key: { to_table: :users }
+  end
+end

db/migrate/20220527120826_add_index_on_category_slug_and_locale.rb

@@ -0,0 +1,7 @@
+class AddIndexOnCategorySlugAndLocale < ActiveRecord::Migration[6.1]
+  def change
+    add_column :categories, :slug, :string, null: false, default: ''
+    add_index :categories, [:slug, :locale, :portal_id], unique: true
+    change_column_default :categories, :slug, from: '', to: nil
+  end
+end