Remove unused javascript fixtures
Improve model validations and specs

* Validate the presence of the model, not the foreign key
* Remove low-value methods and tests: `Receipt#sign_out!`,
  `Receipt#deliverer` (via delegation)
* Specify `inverse_of` on `Delivery` and `Receipt` so `Receipt` can
  validate the presence of `delivery` on creation.
* Improve parsing of `date` in `Delivery.delivered_on`
* Remove `Recipient` model
* Remove 'global' `let` statements in model specs
* Remove `let` statements where not necessary `ability_spec.rb`
Use `Time.zone.today` over `Time.zone.now.to_date`
Cleanup factories

* Sample from Delivery::Deliverers to set the deliverer in a factory
* Remove hacks around setting `delivered_on` in a factory. Use `Timecop`
  in specs when trying to modify `created_at`, `delivered_on` values.