Remove aggressive floating on inline offer fields

The discount edit form works some dark magic to render the offer type selection radio buttons element and get the offer's fields to appear to the right of the radio buttons list. This aggressive floating causes issues for offer types that have a large number of fields, such as the BOGO offers enabled by https://www.drupal.org/project/commerce_discount_extra. It's also just unnecessary.

Instead, we can wrap all offer fields in a div and float that to the left to ensure proper positioning alongside the offer type selection radio buttons. Unfortunately, IEF currently depends on field elements being in the top level of the entity form, so we can't just attach the fields to a container. Instead we have to use a bit of a hack to wrap them using markup elements with appropriate weights.