Add receipt information on deliveries/index
Use attributes_for instead of build
Rename recipient_id and worker_id to user_id

Gets rid of oddities in ember and allows us to better leverage
ember-data.

Intead of manually setting `recipientId` on `Besko.Receipt`, ember-data
will serialize `Besko.Receipt#user` into `user_id` for us. This also
removes the need to translate `Delivery#worker` and `Receipt#recipient`
into `#user` in the serializers.
Remove N+1 query in deliveries/index
Add permissions for deleting deliveries
Allow admins to delete deliveries in controller
Add button to delete deliveries