Add support for arbitrary line item codes

[lyyder]

Update 2019-04-05: The booking breakdown layout has been improved for a more visible sub total row, see updated screenshots

With custom pricing line items can have any sort of code as long as it begins with line-item/.

This PR loosens the line item code validation and changes booking breakdown so that all unknown line items are rendered too.

About rendering unknown line items, initial plan was to render quantity based line items on top of subtotal and the percentage based items below it:

Car wash        $20
Car cleaning    $10
Sub total       $30
Discount       -$5
Commission     $4
---
Total           $29

However, this design got a bit tricky with refunds. Previously the refund has been right after sub total and commissions have their own refund line. If percentage based unknown line items where below the sub total, should they have their own refund line in the breakdown. The simplest solution was to add everything except commissions above the sub total, refund matching those lines below sub total and commissions as they are. The main upside of this design is that the refund always matches the sub total which makes it clear to read.

So current the current design is:

And with refund:

The downside of the design is that unknown line items only present the human readable code and line total. If a line item has quantity other than one for example, it would be nice to be able to read from the breakdown. Maybe one way to show it would be:

2 x Car wash ($20)      $40

[Gnito]

I added a couple of small comments.
Have you tested this on staging against Joe Dunphy's transactions? (I wonder if they change with this PR.)