[deploy] Fix badges slug migration (#7392)

db/migrate/20200420130910_add_unique_index_to_badges_slug.rb

@@ -1,7 +1,15 @@
 class AddUniqueIndexToBadgesSlug < ActiveRecord::Migration[5.2]
   disable_ddl_transaction!
 
-  def change
+  def up
+    return if index_exists?(:badges, :slug)
+
     add_index :badges, :slug, unique: true, algorithm: :concurrently
   end
+
+  def down
+    return unless index_exists?(:badges, :slug)
+
+    remove_index :badges, :slug
+  end
 end