Remember recipients across page refreshes
Remove guard completely

Easier to work in web inspector in browser and write changes to files
separately.
Hide deliveries table if empty

Extracts '.empty-message' and '.empty' classes for easier reuse.
Remove `render_page_to_png` from accounts spec
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