Added support for Avery 3666 labels

[jfrcom]

Thanks for your work on the label template. As I had Avery 3666 labels lying around, I added support for those.

[tmaier]

I think to support multiple labels properly, we need to refactor this a bit more.

I think we would need to create an object, which describes the key dimensions of each label, like margin from page border, width, height, number of rows, number of columns, vertical/horizontal margin between labels.

[jfrcom]

Totally on the same page with you. Honestly, my first go was just a quick and dirty job because I needed those labels printed. I've gone back to the drawing board and refactored the code once more. Alpine.js is new territory for me, so if you've got any pointers on how to do things better, let me know.