CSS pre-processing

[mike42]

We currently use a static CSS file, but this has two main problems-

• Repetition: We will soon generate more height/width scaling classes, which is hard to maintain manually, but can be done easily with a few loops in Sass or similar.
• Vendor prefixes: Output via wkhtmltopdf did not work as expected due to the lack of -webkit-* vendor tags for scaling. This can also be added via a pre-processor.

[mike42]

Example of issue below- correctly-rendered receipt in any modern browser, while wkhtmltopdf does not pick up the height/width multiples-

We need to support this as a target so that we don't need to touch PDF support ourselves.