Add migration to change columns type of attachments

As suggested in the paperclip update discussed in the previous commit,
we force the type of these fields to be bigint.

db/migrate/20200908084257_change_attachment_size_fields_to_bigint.rb

@@ -0,0 +1,7 @@
+class ChangeAttachmentSizeFieldsToBigint < ActiveRecord::Migration[5.1]
+  def change
+    change_column :site_customization_images, :image_file_size, :bigint
+    change_column :images, :attachment_file_size, :bigint
+    change_column :documents, :attachment_file_size, :bigint
+  end
+end