Adds item removal from receipt

* Fix wrong argument to checkout sub-functions.
Implement rest of receipt finishing.

* Marked sell time as not required in admin form.
* Fixed (receipt total) aggregation result when result set was empty.
* Items are marked as sold when receipt finishes.
* Subtotal, Cash and Return displayed in receipt view.
* Receipt is cleared when new receipt starts instead of at end of
previous.
* Adds rounded values to Subtotal and Return.